We studied the relationship between renal hemodynamic changes induced by a single acute administration of captopril (50 mg p.o.) and angiotensin-converting enzyme (ACE) gene insertion/deletion (I/D) polymorphism in 27 healthy human volunteers, 7 with DD genotype, 10 with ID, and 10 with II genotype. The increase in effective renal plasma flow (p < 0.02) and the fall in renal vascular resistance (p < 0.01) in response to captopril were significantly less in subjects with the DD genotype than in subjects with the other genotypes. These data suggest that intrarenal ACE inhibition by captopril differs according to ACE gene ID polymorphism in healthy humans.