The aim of this study was to investigate the cardiovascular effects of exogenous cortisol in fetal sheep, (a) between 100 and 120 days of gestation when cortisol production is minimal and (b) after 130 days when endogenous plasma cortisol starts to rise. Chronically cannulated ovine fetuses (103-120 days, n = 9; 130-137 days, n = 7), received sequentially a 24 h infusion of vehicle (0.9% sodium chloride) and a 24 h infusion of cortisol at 100 micrograms/h. Blood pressure and heart rate changes to bolus injections each of angiotensin II and noradrenaline (0.2, 0.5, 1.0, 2.0 micrograms) were measured before and after the saline and cortisol infusions. Fetuses in each age group, served as additional controls receiving 48 h saline infusions. In both immature and mature age groups, the cortisol infusion increased basal fetal blood cortisol concentrations by 33.7 and 35.4 nmol/l respectively. In the immature group, cortisol, but not saline, caused significant 14.3 and 15.3% increases in basal systolic and diastolic pressures respectively. Basal blood pressure was higher in the mature group, but did not increase further despite the increase in cortisol levels. Furthermore, vascular responsiveness to angiotensin II but not to noradrenaline was significantly enhanced following the cortisol infusion, at both ages. Fetal heart rate did not change following the cortisol infusion. Exogenous cortisol contributes to the regulation of fetal blood pressure in the immature fetus, when other mechanisms have not developed. Cortisol might achieve this, in part, by enhancing vascular sensitivity to angiotensin II.