The 24-hour urinary excretion of prostaglandins (PGs) E2 and F2 alpha, plasma renin activity (PRA), and aldosterone were measured in 7 normal subjects and 8 patients with essential hypertension before and after 5 days of a diet containing less than 20 mmol/day of sodium. Subsequently, sodium was infused intravenously for 4 h (77 mmol/h), and urinary PGs measured in hourly urine collections. Plasma renin activity and aldosterone were measured at hours 2 and 4. In the normal subjects, PGE2 increased significantly (p less than 0.0005), with the diet, while PGF2 alpha did not change. The E/F ratio was increased significantly (p less than 0.01). In contrast, in the hypertensive patients no changes were observed in PGs excretion or in the E/F ratio. PRA and aldosterone were in the normal range after the diet in 4 patients and low in the other 4. Apart from PRA and aldosterone levels, there were no differences between these two subgroups of patients, with regard to change in body weight, blood pressure or electrolyte excretion. PG excretions were also similar. With the Na infusion, PGs returned to control values in the normal subjects. In the hypertensive patients, PG excretion decreased to almost undetectable levels. This coincided with the phenomenon of exaggerated natriuresis. It is concluded that: (1) in hypertensive subjects PGs synthesis is not influenced by chronic changes in the volume status, and (2) renin secretion is relatively independent of PG production in the hypertensive subjects. We suggest that the changes in PG synthesis in hypertension are possibly related to altered sodium handling by the hypertensive kidney.