Urinary excretion of prostaglandin E was measured radioimmunologically in 19 healthy persons (15 men and 4 women) and in 16 patients (10 men and 6 women) with essential hypertension before and after the administration of furosemide. The excretion rates were increased from 26.2 +/- 3.0 to 64.5 +/- 11.3 ng/hr in the former and from 11.9 +/- 2.7 to 26.9 +/- 8.5 ng/hr in the latter. There was a significant difference between them, healthy subjects showing a greater increase than patients with essential hypertension. There was an obvious sexual difference in urinary excretion of prostaglandin. In men, greater increase in the excretion rates was found than in the women. Greater increases were also obtained in healthy men than in hypertensive men and in healthy women than in hypertensive women. The present results suggest that furosemide enhances urinary excretion of prostaglandin E by mechanisms which entails either an increase in prostaglandin synthesis or a decrease in renal metabolism.