Ranitidine is a potent histamine H2-receptor blocker that inhibits histamine- and pentagastrin-induced gastric acid secretion. After doses of 100 mg both intravenously and orally ranitidine kinetics and bioavailability were investigated in a single dose two-way crossover study in 12 normal men. Serum concentrations of ranitidine were determined by radioimmunoassay and urine concentrations by an ion-pair HPLC method. Intravenous data were fitted to exponential equations with the computer program NONLIN; model-independent kinetic parameters were calculated. Elimination t 1/2, plasma clearance, renal clearance, hepatic clearance, and volume of distribution for ranitidine after intravenous injection were 2 hr, 10.4 ml/(min X kg), 7.2 ml/(min X kg), 3.1 ml/(min X kg), and 1.82 l/kg, respectively; after oral doses mean t 1/2 was 2.7 hr and mean bioavailability was 52%. The average cumulative urinary excretion of ranitidine as percent of dose was 69.4 +/- 6.1% and 26.7 +/- 7.2% after intravenous and oral doses.