1. In order to assess the effects of atrial natriuretic factor on the renal biosynthesis of prostaglandins (PG), the urinary excretion of PGE2, PGF2 alpha, 6-keto-PGF1 alpha and thromboxane (Tx)B2 were followed in eight salt-loaded healthy volunteers infused for 2 h with a non hypotensive dose of human atrial natriuretic peptide (hANP, 0.7 nmol min-1). 2. Within 1 h, hANP, infusion produced a marked increase in the urinary PG output, especially of PGE2 and 6-keto-PGF1 alpha (188 +/- 21% and 202 +/- 24% of the pre-infusion values respectively), followed by a significant decrease during the recovery period. 3. No correlations could be uncovered between the urinary excretion of sodium and that of any of the PGs. In contrast, during the infusion of hANP, the urinary output of PGE2 and of 6-keto-PGF1 alpha was found positively related to the urinary flow rate (r = 0.42; P less than 0.05; n = 32 and r = 0.43; P less than 0.05; n = 32 respectively) as well as during the recovery period (r = 0.66, P less than 0.001; n = 32 and r = 0.55; P less than 0.01; n = 32 respectively). 4. It was concluded that, in man, infusion of a non hypotensive dose of hANP is followed by a rise in urinary PG excretion presumably reflecting enhanced renal PG biosynthesis. This increased urinary PG excretion does not seem to be involved in the natriuretic action of hANP but might participate to its diuretic effect.