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1. Changes in cortisol and corticosterone output and blood flow from the adrenal gland have been determined in the conscious unrestrained calf during I.V. infusions of synthetic adrenocorticotrophin (Synacthen) at 0–5 ng‐kg(∓1) min‐ minus 1 (low dose), 5 ng‐kg‐ minus 1 (medium dose), 50 ng‐kg‐ minus 1 min‐ minus 1 (high dose) and 500 ng‐kg‐ minus 1 min‐ minus 1 (medium dose), 50 ng‐kg‐ minus 1 min‐ minus 1 (high dose) and 500 ng‐kg‐ minus 1 min‐ minus 1(very high dose). 2. Infusions at the low dose produced a rise in adrenal output of both cortisol and corticosterone to maximum values of approximately 100 and 30 ng‐kg‐ minus 1 min‐ minus 1 respectively. Mean output of both steroids was significantly increased within 5 min, reached a maximum within 10 min and had fallen to resting levels 10 min after the infusion was discontinued. 3. The effects of infusions at both the high and very high doses were closely similar; maximal cortisol outputs were within the range 600–800 ng‐kg‐ minus 1 min‐ minus 1 and corticosterone 350–500 ng‐kg‐ minus 1 min‐ minus 1 in both groups. 4. When the infusions were terminated, pronounced differences were observed in the rates at which steroid outputs declined. Basal levels were restored within 10 min following the low dose and within 60 min in medium dose animals, but both cortisol and corticosterone output were still elevated 2 hr after infusion in high dose animals. In calves infused at the very high dose, cortisol output did not fall significantly during 2 hr period. The ratio of cortisol: corticosterone released from the adrenal gland immediately before infusion (3–2 +/− 0–3) approximated to the proportions in which the two steroids were found in the arterial plasma, but fell progressively to a minimum (1–3 +/− 0–1) with increasing doses of Synacthen. Conversely, the ratio of the steroids in the arterial plasma was increased during infusions at the low dose, but not at the higher doses. 6. No significant change in adrenal blood flow occurred during Synacthen infusion in low dose animals despite the increase in steroid output. In medium dose animals blood flow through the gland rose during infusion by approximately 75 per cent while in both high and very high dose animals the flow increased by up to 300 percent. 7. In the three groups in which adrenal hyperaemia occurred, blood flow had fallen to within the resting range 45 min after infusion: in each case this fall was much more rapid than the fall in steroid output. No significant increase in aortic blood pressure or heart rate accompanied infusion of Synacthen, indicating that adrenal hyperaemia was dependent upon vasodilatation with the gland. 8. Administration of cycloheximide (10 mg/kg) by I.V. injection either before or during an infusion of Synacthen, inhibited steroidogenesis without affecting the vasodilator response.

1. Variations in the output of glucocorticoids and catecholamines from the right adrenal gland, in response to insulin hypoglycaemia, have been investigated in calves 2‐5 weeks after birth. These have been correlated with changes in the concentration of glucocorticoids and glucagon in arterial plasma. 2. Moderate hypoglycaemia for a limited period (0–1 u. insulin/kg), elicited a prompt increase in steroid output from the adrenal gland followed by a significant rise in plasma glucagon concentration. By comparison, changes in both catecholamine output and peripheral plasma glucocorticoid concentrations were found to be trivial in this group of animals. 3. Administration of a larger dose of insulin (0–5 u./kg) produced a more substantial fall in plasma glucose concentration followed by spontaneous recovery within 2‐3 hr. This stimulus elicited the release of greater amounts of both cortisol and corticosterone, followed by a significant increase both in the output of adrenaline and in plasma glucagon concentration. Increase in steroid output was accompanied by an increase in adrenal blood flow and was associated with elevated concentrations of both steroids in arterial plasma. 4. The adrenal cortical response and associated changes in plasma steroid concentration were found to be transient even in response to persistent and intense hypoglycaemia (4 u. insulin/kg). The increase in plasma glucagon concentration in this group of animals was not significantly greater than that produced by smaller doses of insulin. However, substantial amounts of adrenaline (78 plus or minus 14 ng. kg‐minus 1 min‐minus 1; maximum; n equals 9) together with a little noradrenaline (10 plus or minus 3 ng.kg‐minus 1 min‐minus 1; maximum; n equals 9) were released from the right adrenal gland under these conditions. 5. Changes in adrenal blood flow could be related to adrenal glucocorticoid output in calves given 0–1 or 0–5 u. insulin/kg. In animals given the largest dose of insulin adrenal blood flow was found to increase coincidentally with rising steroid output but this hyperaemia then persisted after steroid output had subsided to values within the normal range. 6. Calves given the largest dose of insulin (4–0 u./kg) invariably collapsed and convulsed after 2‐3 hr, but these symptoms could not be related to any particular endocrine response. No clinical signs of hypoglycaemia were observed in the other animals. 7. The results are discussed in relation to previous studies of adrenal function in this and other species.

1. Plasma concentrations of cortisol and corticosterone measured by competitive protein binding in rats between 5 and 28 days after birth have been related to the intestinal uptake of [125I]polyvinyl pyrrolidone (PVP).

1. Bilateral adrenalectomy in 18‐day‐old rats resulted in an extension by approximately 4 days of the period during which the villous epithelial cells of the small intestine took up polyvinyl pyrrolidone (PVP) of mean mol. wt. 160,000.

1. Plasma aldosterone concentrations have been measured by radio‐immunoassay in the new‐born and adult guinea‐pig and related to plasma cortisol and corticosterone concentrations.

Plasma cortisol and corticosterone were separated by Sephadex LH-20 chromatography. The high concentration of plasma cortisol immediately after birth in the guinea-pig coincided with the decline in 125I-labelled polyvinyl pyrrolidone (PVP) uptake (closure) between 48 and 72 h. Similarly, in the rabbit, plasma cortisol concentration was increasing at the time of closure, 18–21 days after birth. These results suggest that there is a correlation between the time when the concentration of the main plasma adrenocortical steroid rises and the time when the neonatal small intestine ceases to take up PVP.