Intravenous vasopressin (1-3 mu-units min-1 kg-1) had an antidiuretic effect on water-loaded man and also diminished potassium excretion. As noted by others, aspirin (2.4 g) enhanced the antidiuretic effect of vasopressin, but the fall in potassium excretion was not modified by prior administration of aspirin, which makes it unlikely that the fall was due to the release of endogenous prostaglandins. After terminating the infusion of vasopressin, the fall in potassium output persisted longer than the antidiuresis, which makes it unlikely that the antikaliuretic effect of vasopressin is secondary to its effect on urine flow. The unchanged antikaliuretic effect of vasopressin after aspirin treatment, together with its persistence after terminating the infusion, suggest the possible existence of vasopressin-mediated potassium absorption in the distal nephron in certain circumstances. Aspirin administration had specific effects of its own in water-loaded man. It decreased both the water diuresis and sodium excretion but did not alter potassium excretion or urine osmolality.