We have investigated the pharmacokinetics of 14C-labeled diltiazem, 20 mg, given as an i.v. infusion over 20 min in 10 healthy volunteers. This disposition of the drug could be described using a two-compartment model with half-lives of 0.40 +/- 0.48 h (mean +/- SD) in the alpha phase and 2.77 +/- 0.82 h in the beta phase. Systemic clearance was 992 +/- 159 ml/min; the volume of the central compartment was 119 +/- 77 L, and the volume of distribution at steady state was 209 +/- 56 L. The concentrations of metabolites (deacetyldiltiazem, N-demethyldiltiazem, and N-demethyl-deacetyldiltiazem) were low, and no pharmacokinetic parameters for these could be calculated. The median cumulative excretion of radioactivity during 120 h was 87.3%. The drug was mainly excreted in urine (71.1 +/- 7.8%), and the remaining amounts was excreted in feces. There were slight but significant decreases in supine systolic and diastolic blood pressures and heart rate. The PQ interval was significantly prolonged for 5 h, and in multiple regression analyses there were good correlations (p less than 0.01) between PQ intervals and logarithms of plasma concentrations of diltiazem.