1. To clarify the mechanism of sex-dependent and independent kidney secretion of major nilvadipine metabolites (3, 7) in rat, renal clearance corrected for protein binding and glomerular filtration rate (GFR) was measured in both sexes. The effect of probenecid, an inhibitor of organic anion transport, on these measurements was also investigated. 2. Clear sex-dependent active secretion was observed in the renal excretion of 3 (3-carboxylic acid pyridine derivative). In the female rat, 3, clearance was approximately 32-fold greater than GFR and was markedly decreased by probenecid. Conversely, in the male rat, renal clearance of 3 was only a fraction of GFR and was unaffected by probenecid. 3. Sex-independent active secretion was observed in the renal excretion of 7 (5-carboxylic acid pyridine derivative). In both sexes of rat 7 clearance was about 22-fold greater than GFR and was markedly reduced by probenecid. 4. A clear presence of sex-dependent and independent active secretion mechanisms in the kidney has been demonstrated in rat. The female rat is able to eliminate 3 and 7 in urine by an active secretion mechanism that is inhibited by probenecid. In the male rat, a transport mechanism for 7 is present, but either lacks or is apparently inactive for 3.