The renal clearance of inulin (Clin), 14C-creatinine (Cl14C.cr) and endogenous creatinine (Clcr) were measured in 6 healthy subjects before and after a single intravenous bolus of cimetidine (5 mg/kg). The initial Cli4C.cr/Clin ratio (mean ± SD) was 1.68 ± 0.17; this fell to a nadir of 1.35 ± 0.18 soon after cimetidine was infused and rose again as the drug was excreted. This is the first cationic drug to show substantial inhibition of tubular secretion of creatinine in huma