The purpose of this study was to establish (in the primate) maternal-fetal relations for blood gas parameters and certain related aspects of fetal cardiovascular function with the mother at rest and subjected to a range of altered states of oxygenation. In 10 chronically cannulated rhesus monkey fetuses, with the mother lightly sedated with phencyclidine, on each of two days we studied the effects of maternal hypoxia and hyperoxia [(15, 10, and 100% maternal fractional concentration of inspired O2 (FIO2)] on maternal and fetal pH, PCO2, PO2 and O2 concentration (CaO2) and fetal heart rate, blood pressure, and superior vena caval (SVC) flow distribution. We observed a linear correlation between maternal and fetal pH and PCO2. There was evidence of a fetal metabolic acidosis with 10 but not with 15% hypoxia. Maternal-fetal PO2 and CaO2 correlated in curvilinear and linear fashion, respectively. Maternal-fetal CaO2 difference was maintained within a narrow range (2.7-4.8 vol%) for various states of maternal oxygenation, showing a tendency to expand only with severe (10%) hypoxia. Results indicate that quantitative delivery of O2 to the placenta was of paramount importance as a determinant of O2 transfer to the fetus and are compatible with a "concurrent flow" model of transplacental O2 diffusion. Fetal heart rate decreased progressively with 15 and 10% hypoxia. SVC flow to lungs and placenta varied reciprocally with changes in fetal oxygenation. As indicated by distribution to the heart, shunting of SVC flow through the foramen ovale was modest at rest, with no evident change in response to fetal hypoxemia.