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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% O2 with 3% CO2 in N2. 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 Pa, 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% CO2 with 18% O2 in N2 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% O2 in N2, did not significantly increase foetal breathing movements unless the ewe was in labour. In labour the foetuses had lower Pa, O2 values initially and a reduced incidence of foetal breathing, both of which were increased by maternal hyperoxia.
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