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1. Cerebral blood flow was measured in 17 baboons, anaesthetized with pentobarbitone, paralysed with gallamine and mechanically ventilated and in which the right sinus and both aortic nerves had been cut and the left carotid sinus vascularly isolated. Later in each experiment, the head was artificially perfused with femoral arterial blood via the innominate artery.

1. Respiratory responses were measured in forty‐seven cats, made decerebrate or anaesthetized with pentobarbitone or chloralose‐urethane, to changes in the level of CO2infused into the inferior vena cava via an external oxygenator circuit or to changes in the volume of venous return or to the inhalation of CO2.

1. The blood supply to the medulla was determined by the injection of indian ink via the vertebral arteries. Virtually the whole medulla was supplied by penetrating vessels from the ventral surface. The highest density of small arterioles and venules was found close to the roots of XII and on the ventrolateral surface.

1. A rapidly responding CO2 sensor and cuvette were placed in the carotid artery of anaesthetized cats and changes of PCO2 in post‐pulmonary capillary blood were recorded when flow through the cuvette was suddenly stopped. 2. Under control conditions when the cats breathed spontaneously or were ventilated artificially, stop‐flow caused Pa, CO2 to decay by 2‐‐5 mmHg reaching a new equilibrium in 10‐‐15 sec. The amount by which CO2 decayed was reduced by the inhalation of high O2. The decay was enhanced by hypoxia, the inhalation or infusion of CO2. It was reversed by the administration of acetazolamide: now with stop‐flow, Pa, CO2 rose by 8‐‐15 mmHg. 3. the mechanism of this decay is uncertain. We propose that it is due to the transfer of CO2 from plasma to red cells in post‐capillary blood in response to the reduction of [H+]i as H+ binds to Hb following the oxylabile release of CO2 from Hb.

1. A CO2 sensor using mass spectrometry is described. It responds linearly to O2 and CO2 with a time constant of 0.5 sec: it is not affected by pressure or flow in the physiological range: its temperature coefficient is 4 mm Hg PCO2 per degree C at 37 degrees C. 2. When this sensor and its through flow cuvette were placed in a common carotid artery‐to‐jugular vein loop in anaesthetized cats breathing spontaneously or being ventilated artificially, fluctuations of Pa, CO2 which had the same period as respiration were readily observed. 3. The amplitude of these fluctuations varied inversely with respiratory frequency being less than 1.5 mmHg Pa, CO2 in the range of normal respiratory frequencies in the cat, 25‐‐35 min‐1. The amplitude also varied with the mixed venous‐to‐end tidal CO2 difference which was altered either by giving the cat CO2 to inhale or infusing CO2 intravenously. 4. We have concluded that these fluctuations of Pa, CO2 are unlikely to provide a significant drive to ventilation at normal respiratory frequencies but they may provide a signal that gas exchange in the lung is less than optimal.

1. Afferent activity was measured in the vagus nerves in sixteen foetuses within 2–3 days of natural term and in two foetuses 130 and 135 days gestational age, exteriorized by hysterotomy from seventeen ewes given either a spinal anaesthetic (ten) or pentobarbitone sodium (seven).

1. In the sinus nerve of mature foetal sheep (120–147 days gestational age) three types of nerve activity have been demonstrated: (a) baroreceptor afferent, in which the nerve activity was synchronous with the pulse pressure wave and which followed pressure changes in the carotid sinus including those caused by compressing the umbilical cord. The discharge of nine out of twelve baroreceptor afferents increased with sympathetic stimulation, (b) a subtype of (a) in which there was irregular activity from pressure receptors which most probably represented baroreceptors discharging at or about threshold, and (c) in the older foetuses (130–147 days), there was activity which was unaffected by changes in carotid sinus pressure, Pa, O2or NaCN but which increased in discharge rate with compression of the umbilical cord or with stimulation of the cervical sympathetic; this activity then increased with injection of NaCN and was thus identified as chemoreceptor in origin. In addition, a pathway originating in the cervical sympathetic and which passed through the superior cervical ganglion was demonstrated by evoking a compound action potential on stimulation of the preganglionic cervical sympathetic nerve.

1. Respiratory responses to altered Pa, O2at constant Pa, CO2and altered Pa, CO2at constant Pa, O2were measured in fourteen unanaesthetized ducks 5‐9 weeks old.

1. The anatomy of the carotid body and its afferent nerve supply was studied in the duck and a method of denervating the carotid body which ensures a satisfactory post‐operative course is described.

1. Carotid body blood flow (c.b.f.) and carotid arterial—carotid body venous oxygen (A—V O2) difference were measured and carotid body oxygen consumption calculated in twenty‐six cats anaesthetized with pentobarbitone sodium, paralysed with gallamine triethiodide and ventilated mechanically.

1. Carotid body blood flow (c.b.f.), the arterio—venous oxygen (A—V O2) difference and oxygen consumption were measured in forty‐seven cats, anaesthetized with pentobarbitone, paralysed with gallamine and ventilated artificially. Carotid sinus and cervical sympathetic nerves were intact throughout.

1. The responses of single afferent fibres of carotid body chemoreceptors to independent changes in arterial O2and CO2tensions and pH were studied in the cat in vivo.

1. Activity in forty‐two single chemoreceptor afferent fibres from the carotid body in thirty‐nine cats was measured when the carotid body was naturally and artificially perfused. In nine of these cats, carotid body venous flow was also measured.

1. The activity in carotid body chemoreceptor afferent fibres in the cat has been recorded and found to have a rhythm with the same period as respiration.

1. Parallel recordings have been made of the cervical sympathetic and carotid body chemoreceptor activity in the anaesthetized cat.

1. Tidal volume, carotid artery oxygen tension (Pa,O2) and blood pressure and chemoreceptor activity in the sinus nerve have been continuously measured and recorded in nine lambs anaesthetized with pentobarbitone sodium, and varying in age from some minutes after birth to 5 days after birth.

1. Recordings have been made of the activity in carotid body chemoreceptor afferent fibres and in the cervical sympathetic of the cat during passive movement of the hind limbs.

1. The respiratory response to inhaled CO2was measured in twenty unanaesthetized new‐born lambs aged 4 hr‐10 days. Measurement of resting arterial pH, PCO2and plasma bicarbonate showed a non‐respiratory acidosis immediately after birth which was corrected in the first 24‐28 hr: thereafter, the acid—base pattern was of a compensated respiratory alkalosis.

1. Twenty‐seven unanaesthetized new‐born lambs, 6 hr‐10 days old, responded to two levels of inspired oxygen, 125 and 110 mm Hg (alveolar CO2being controlled) with a sustained increase in minute ventilation (V̇), a small increase in heart rate and a less consistent rise in systemic blood pressure.

1. Methods are described for measuring tidal volume and frequency, end‐tidal CO2, blood pressure and heart rate, and arterial gas tensions in the unanaesthetized new‐born lamb.