Weanling male rats were fed an essential fatty acid (EFA)-deficient diet for 25 weeks and then switched to an EFA-supplemented diet for 3 weeks. Control rats received the EFA-supplemented diet for 25 weeks and then the EFA-deficient diet for 3 weeks. Throughout the last 19 weeks, the rats were housed in metabolic cages once a week for a 24-hr period. Urinary excretion of prostaglandin E (PGE) was estimated by radioimmunoassay. Throughout a period of 12 weeks (weeks 13-24) water consumption increased ca. 60%, and urine output and PGE excretion decreased ca. 45% and 70%, respectively, in the EFA-deficient rats. Feeding EFA-supplemented diet to the EFA-deficient rats for 3 weeks decreased the water consumption and raised the urine output to that observed in the controls. However, the urine output was corrected within 1 day whereas the water consumption was not corrected until the second measurement 8 days after the dietary change. The PGE excretion increased more than 9-fold (from 18±8 ng/24 hr to 165±51 ng/24 hr) 1 day after EFA-supplementation, followed by a decrease to 86±29 ng/24 hr over the following 2 weeks. On the basis of the present data, it is suggested that EFA deficiency in rats causes diminished PGE excretion, which can be normalized by EFA supplementation. The normalization of the urine flow may, in part, be caused by the concomitant considerable increase in endogenous PGE synthesis.