Studies were performed in anesthetized monkeys in the third trimester of pregnancy, with continuous monitoring of maternal blood pressure and uterine artery blood flow by electromagnetic flow probe and with measurement of prostaglandin levels in uterine vein samples. Acute elevation of the systemic blood pressure by intravenous infusion of angiotensin II was associated with increased uterine artery blood flow and an increase in the levels of prostaglandin E measured in the uterine vein. An increase in PGE levels was also observed following the discontinuation of the angiotensin II infusion, when blood pressure and uterine artery blood flow returned to baseline levels. Intravenous administration of indomethacin was associated with a slight rise in maternal blood pressure. The changes in uterine artery blood flow following indomethacin treatment were variable, perhaps reflecting the stresses of surgery and anesthesia. In single experiments, intravenous diazoxide produced a profound hypotension, and intravenous furosemide induced a prompt and marked diuresis, associated with a very marked fall in uterine artery blood flow.