The urinary excretion rate of prostaglandin E2 (PGE2) and prostaglandin F2 alpha (PGF2 alpha) was determined in patients with the nephrotic syndrome both before and after an oral water load in 21 patients and 17 control subjects, and before and after remission of the syndrome in 8 of the patients. In the nephrotic syndrome PGE2 excretion rate varied considerably during basal conditions, remission was accompanied by an increase in the PGE2 excretion, and both basal PGF2 alpha excretion rate and the normal response in PGF2 alpha water loading were reduced. A significant, positive correlation was found between urine flow rate and excretion rate of PGE2 in the periods with the largest urine flow rate in both patients and control subjects. It is suggested that a relatively suppressed renal prostaglandin production may be a pathogenetic factor for sodium and water retention in the nephrotic syndrome, although it cannot be excluded that the abnormal prostaglandin excretion pattern is secondary, at least partially, to the reduction of urine flow rate.