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Fifty-eight adults, nearly all male, aged between 35 and 50, with slight bronchiopneumopathy in a state of functional compensation, were subjected to hypoxia by rerespiration. Heart rate and oxyhaemoglobin saturation were observed. In nearly all cases, hypoxia caused an increase in heart rate that varied considerably from subject to subject, but seemed more marked between the 1st and 2nd minute after start of the trial. Normalization of heart rate, similarly very variable, reflected the increase phase and was complete in periode varying between 30 and 210 seconds. It may be supposed that as regards heart rate the wide variety of responses to the same stimulus is due to individual states of equilibrium of a predominantly vegetative nature. There was no relation between duration of the hypoxia trial, extent of desaturation and time of resaturation in air because the rate of haemoglobin desaturation and resaturation varied from case to case.