We have shown that the inhibition of prostaglandin (PG) synthesis in man decreases the fractional clearance of urea (FCurea). To understand the mechanism(s) by which PG affect the renal handling of urea, 6 normal volunteers were randomly studied in maximal antidiuresis (by water deprivation and by administering 1-desamino-8-.D-arginine vasopressin) before and during PGE1 infusion, in two separate occasions: (A) after 7 days of normal protein (1 g/kg b.w./day) and water intake (10 ml/kg b.w./day), and (B) after 7 days of low protein intake (0.5 g/kg b.w./day) and high water intake (80 ml/kg b.w./day) to lower the corticomedullary osmotic gradient. During infusion of PGE1 at rates of 0.01, 0,05 and 0.1 μg/min/kg, randomly administered, the urinary fluid losses were replaced by infusing equal volumes of hypotonic NaCl (80 mmol/l). To evaluate the time effects of this protocol, control studies were performed in an other 8 subjects receiving vehicle infusion without PGE1· In study A, FCurea rose by 23% (p < 0.01) at the lowest PGE1 infusion rate (0.01 μg/min/kg), in the absence of any simultaneous change in water and salt output, Uosm, PAH and inulin clearance. Higher PGE, infusion rates (0.05 and 0.1 μg/min/kg) were associated with a progressive increase of FCurea (50%, p < 0.001 and 91%, p < 0.001, respectively), fractional clearance of water and salt output, inulin and PAH clearance and reduced Uosm from 1,005 (22 SEM; basal value) to 772 (38 SEM; minimum value) mosm/kg (p < 0.001). In study B, the basal value of Uosm was 762 (22 SEM) mosm/kg, markedly lower than the basal value of study A (p < 0.01); in this condition, the increasing infusion rates of PGE1 caused the same changes of FCurea and the other parameters as in study A. FCurea was directly related to dose infusion of PGE1 both in study A and B (p < 0.001). The slopes of these two linear regression analyses did not statistically differ. Finally, both FCurea and fractional clearance of water did not show significant changes among the several periods of the control studies. We conclude that in human subjects, the inhibition of urea tubular reabsorption, observed during PGE1 infusion, is: (1) not associated with change in tubular handling of salt and water at the lowest infusion of PGE1; (2) not mediated by passive hydrosmotic forces or by antagonism with ADH; (3) dependent on the dose of exogenous PGE1.