Since renal prostaglandins may contribute to natriuresis induced by endogenous atrial natriuretic factor (ANF), acute volume expansion (AVL), a known stimulus of ANF and prostaglandins, was induced in 8 healthy women in order to test whether the consequent sodium and water diuresis is altered by prostaglandin inhibition. AVL (i.v. infusion of a 2 liter 5% glucose solution in 1 h) was infused after placebo and after inhibition of prostaglandins with diclofenac (200 mg/day orally for 4 days), in a double blind randomized cross-over fashion. Urinary eicosanoids (PGE2, PGF2 alpha, 6-ketoPGF1 alpha, TXB2--RIA), plasma ANF (RIA) and urinary electrolytes were determined before, during and after AVL under both placebo and diclofenac regimes. During placebo, AVL induced sustained increases in plasma ANF (174% at peak, p less than 0.001 ANOVA), excretion of the four eicosanoids (149%-1172%, p less than 0.005-0.001), urinary volume (UV, 815%, p less than 0.001), natriuresis (UNa, 98%, p less than 0.005) and in kaliuresis (UK, 90%, p less than 0.001). Cyclooxygenase inhibition resulted in a reduction of over 70% in both baseline values and AVL-induced increase of eicosanoids. It did not alter either baseline levels or AVL-stimulated ANF, UV, UNa and UK in relation to placebo. The present results suggest that the diuretic and natriuretic activity of ANF is not mediated by renal PGs in humans.