Blood concentration and urinary excretion of captopril following 50 mg oral administration were determined by high-performance liquid chromatography in normal subjects and patients with chronic renal failure. In normal subjects, the maximum blood concentration of the free form of captopril was obtained within 1 hour and was not detectable after 6 hours; 41% of administered captopril was excreted into the urine as free form and metabolites within 2 hours, and 58% within 6 hours. In chronic renal failure patients with an average serum creatinine of 5.1 mg/dl, the absorption constant (Ka), maximum concentration (Cmax), and area under the blood concentration curve (AUC) were not significantly different from those in the normal subjects, but the elimination constant (Ke) and biological half-life (T1/2) showed a significant delay in the disappearance of captopril from the blood (p less than 0.01 respectively). The cumulative amount of urinary excretion of either free-form captopril or its' metabolites was significantly decreased at 2, 4, and 6 hours in chronic renal failure patients (p less than 0.01 or less, respectively). Impairment of kidney function is suggested to be an important factor in the promotion of blood retention of captopril.