Four healthy men took a single oral dose of 200 mg spironolactone after a standardized breakfast. Blood samples were drawn until 24 hours after dosing. A specific HPLC method was used to determine the serum concentrations of spironolactone and its metabolites 7 alpha-thiomethylspirolactone, 6 beta-hydroxy-7 alpha-thiomethylspirolactone, and canrenone. Pharmacokinetic parameters were derived from the serum concentration-time course of each compound. Spironolactone, 7 alpha-thiomethylspirolactone, and canrenone are known to have antimineralocorticoid activity in man. Our study demonstrated that: (1) 7 alpha-Thiomethylspirolactone is the main metabolite of spironolactone after a single oral dose as judged by the AUC(0-24) and the maximum concentration; and (2) unchanged spironolactone was detected in serum, with a maximum concentration at 1 hour and detectable levels up to 8 hours after dosing. Our findings are contrary to the widely accepted belief that spironolactone is metabolized too rapidly to be detected in serum after oral dosing and that canrenone is the principal metabolite of spironolactone.