To evaluate the safety of prophylactic vasopressor administration prior to obstetric conduction anesthesia, equipotent doses of ephedrine, metaraminol, mephentermine, and methoxamine were administered to 16 nonanesthetized pregnant ewes near term. When the maternal blood pressure was increased by 50 per cent, uterine blood flow was unchanged with ephedrine and was reduced 20 per cent with mephentermine (P gt; 0.05); 45 per cent with metaraminol (P lt; 0.005) and 62 per cent with methoxamine (P lt; 0.05). Metaraminol and methoxamine consistently decreased uterine blood flow at all levels of maternal blood pressure elevation. No significant change in fetal blood gas and acid-base variables was demonstrated.