1. The purpose of this investigation was to determine the effects of maternal hyperoxaemia and hypercapnia on the uterine vascular bed and foetal oxygenation in the large white sow at 80-90 days gestation. 2. When maternal hyperoxaemia was induced with 100% oxygen, there was a highly significant rise in the maternal arterial oxygen tension, but no other significant blood gas or vascular changes were observed. 3. When mild maternal hypercapnia was superimposed on maternal hyperoxaemia (oxygen plus 6% carbon dioxide), the oxygen tension and saturation of both the maternal uterine venous and foetal umbilical venous bloods were found when severe hypercapnia was induced (oxygen plus 50% carbon dioxide) but in this case all blood samples showed dramatic changes in PCO2 and pH. These changes were accompanied by an increase in the systemic blood pressure and uterine blood flow, and a decrease in uterine vascular resistance. 4. When mild hypercapnia was induced without hyperoxaemia (air plus 5% carbon dioxide) significant increases were recorded in the oxygen tension and saturation of uterine venous and foetal umbilical venous bloods. Systemic and uterine vascular resistance fell. 5. It was concluded that the increased foetal oxygen tension during maternal hypercapnia was the result of the increased uterine blood flow and greater mass delivery of oxygen to the placenta, so that once the oxygen requirements of the placental tissues themselves were exceeded there would be an increased oxygen gradient at the site of gas exchange. 6. Carbon dioxide concentration in arterial blood plays an important role in determining blood flow through the pregnant uterus in the sow.