1. In acute experiments, high levels of endogenous prostaglandins, provoked by operative stress, could obscure or alter the actions of infused prostaglandins on the kidney. For this reason we decided to compare the effects of infusing prostaglandin E(1) into the renal artery of the dog before and after the administration of phenylbutazone, a prostaglandin synthetase inhibitor.2. Infusion of prostaglandin E(1) into the left renal artery of the pre-phenylbutazone treated dog undergoing a mannitol diuresis increased renal plasma flow, glomerular filtration rate and the excretion of salt and water. The findings are in general agreement with those reported by others.3. Following phenylbutazone administration the vascular and saluretic actions of prostaglandin E(1) were unchanged but a reduced diuretic effect was observed. The response to a low dose of prostaglandin E(1) (0.05 mug/min) was reduced from 1.46 +/- 0.15 to 0.96 +/- 0.16 ml./min (P < 0.001) and the response to a high dose (0.5 mug/min) from 1.82 +/- 0.19 to 0.99 +/- 0.31 ml./min (P < 0.002).4. A significantly less dilute urine was excreted during prostaglandin infusion in the dog after phenylbutazone treatment than before. The reduction in the diuretic response was of the same order as the decrease in the free water clearance response, while the increase in osmolar clearance was unchanged.5. In water-loaded dogs treated with phenylbutazone, infusion of prostaglandin E(1) into the left renal artery had a biphasic effect on urine output from the left kidney. An initial diuretic response to a low dose of prostaglandin E(1) disappeared with the infusion of higher doses, and antidiuresis developed in the immediate post-infusion period.6. As prostaglandin was infused into the left kidney progressive antidiuresis was seen in the non-infused right kidney.7. It is concluded that endogenous prostaglandins do not obscure or alter the vascular and saluretic actions of intrarenal prostaglandin E(1). The findings question the proposed link between the vascular and saluretic actions of this compound.8. It is suggested that the reduced diuretic effect of prostaglandin E(1) in series no. 1, and the antidiuresis in the water-loaded dogs, are caused by the release of endogenous ADH. It is further suggested that phenylbutazone unmasks this release by removing the endogenous prostaglandins. If these deductions are correct, the findings support the anti-ADH role assigned to endogenous prostaglandins by Anderson, Berl, McDonald & Schrier (1975).