The pharmacokinetics and pharmacodynamics of intravenous furosemide, 40 mg, were studied in four healthy male subjects in a crossover fashion with and without probenecid pretreatment. In each study, 16 plasma and 10 urine samples were collected over 24 hr. Fluid and electrolyte urinary losses were replaced orally throughout the study. Unchanged furosemide and probenecid were measured using high-pressure liquid chromatography; urinary sodium was measured by flame photometry. Although probenecid caused marked changes in the pharmacokinetic parameters of furosemide (increased area under the curve, decreased plasma and renal clearance, increased half-life, and decreased fraction excreted unchanged in the urine), there was no significant difference in its gross 8-hr natriuretic and diuretic effect. However, analysis of the time course of natriuresis showed a pattern similar to that of the urinary furosemide excretion rate, whereas the plasma concentration was poorly correlated over the entire dose-response curve.