1 Ranitidine single dose pharmacokinetics and absolute bioavailability have been studied in five healthy male volunteers. Following an overnight fast, 150 mg was given intravenously as a bolus injection or orally as a tablet formulation to each subject on separate occasions. 2 Following intravenous administration, plasma levels declined biexponentially. The mean (+/- s.d.) distribution half-life (t 1/2 alpha) was 6.6 +/- 1.6 min; plasma half-life (t 1/2 beta) was 1.7 +/- 0.2 h; the volume of distribution (V) was 96 +/- 9 1; total body clearance (CL) was 647 +/- 94 ml/min and renal clearance (CLR) 520 +/- 123 ml/min. 3 Following oral administration plasma levels showed a bimodal pattern with a first peak at 1.1 +/- 0.4 h and a second peak at 3 +/- 0 h. The absolute availability was 60 +/- 17%. The plasma half-life (t 1/2) of 2.3 +/- 0.4 h was significantly longer (P less than 0.05) after oral than after i.v. administration. 4 Renal excretion of unchanged ranitidine accounted for 79 +/- 9% of the dose after i.v. administration and for 27 +/- 7% after oral administration. 5 Our results suggest a more extensive biotransformation of ranitidine and biliary excretion of metabolites after oral administration while i.v. administration ranitidine is preferentially excreted unchanged in the urine.