1. Foetal breathing movements, electrocortical activity, arterial pressure and heart rate were recorded continuously in chronically catheterized sheep, 97-145 days pregnant.2. With increasing gestational age there was a fall in heart rate of 0.67 beats/day and a rise in arterial pressure of 0.46 mmHg/day.3. Hypoxaemia in the foetus was induced by allowing the ewe to breathe low oxygen mixtures, 9% O(2) with 3% CO(2) in N(2). In the younger foetuses there was an initial rise in heart rate whereas in the older foetuses there was a fall. After the end of hypoxia there was a persistent tachycardia in both groups. In the older foetuses there was a rise of arterial pressure.4. Two vagotomized older foetuses showed cardiovascular responses similar to those of the younger foetuses.5. Foetal breathing movements were abolished by hypoxaemia in twenty-two of twenty-five experiments. In the three exceptional experiments there was a small rise in P(a, CO2).6. The proportion of time occupied by low voltage electrocortical activity in the foetus was reduced by hypoxaemia.7. Hypercapnia was induced by giving the ewe 4-6% CO(2) with 18% O(2) in N(2) to breathe. After an initial slight fall the foetal heart rate increased and there was a small rise in foetal arterial pressure.8. The proportion of time occupied by low voltage electrocortical activity and breathing movements was increased by hypercapnia.9. Maternal hyperoxia, induced by giving 50% O(2) in N(2), did not significantly increase foetal breathing movements unless the ewe was in labour. In labour the foetuses had lower P(a, O2) values initially and a reduced incidence of foetal breathing, both of which were increased by maternal hyperoxia.