The changes in regional cerebral blood flow (CBF) in response to prolonged hypoxaemia were measured using coloured microspheres in the 0.6-gestation ovine fetus (n = 5). Fetal hypoxaemia was induced for 12 h by reducing maternal uterine blood flow with an adjustable clamp. CBF (mL min-1 100 g-1) was increased (P < 0.05) from control values (38.7 +/- 3.5) to 105.6 +/- 5.6 at 6 h of hypoxaemia, and to 121.9 +/- 23.1 at 12 h of hypoxaemia. One hour after fetal hypoxaemia had ceased, CBF (54.0 +/- 3.3) had decreased (P < 0.05) towards control values indicating incomplete cardiovascular recovery. Cerebral vascular resistance at 6 h and 12 h of hypoxaemia was lower (P < 0.05) than control values, and returned to control values 1 h after fetal hypoxaemia had ceased. Cerebral oxygen delivery at 6 h and 12 h of hypoxaemia was not significantly different from control values, but was higher (P < 0.05) 1 h after hypoxaemia had ceased. It is concluded that CBF is sufficiently increased during prolonged hypoxaemia in the mid-gestation fetus to maintain cerebral oxygen delivery.