We studied the relationship of fetal and maternal vascular pressures to umbilical and uterine blood flow in the unanesthetized ewe and in the sheep fetus in utero by placing electromagnetic flow transducers around both the common umbilical and uterine arteries. Reductions in umbilical arterial pressure or elevations in umbilical venous pressure decreased umbilical blood flow without affecting either the uterine arterial blood flow or other maternal cardiovascular variables which were studied. Elevations in uterine venous pressure or reductions in uterine arterial pressure decreased uterine arterial flow but these interventions had no effect on umbilical blood flow until fetal hypoxemia and bradycardia occurred. When the bradycardia of the fetal hypoxic response was inhibited by atropine, alterations in maternal vascular pressure had no effect on umbilical arterial flow. These data do not support the presence of a "sluice" or "waterfall" effect in the umbilical-placental circulation of the sheep fetus in utero.