Seven healthy volunteers showing a fourteenfold range in steady-state plasma concentrations on oral alprenolol (200 mg b.i.d.) were investigated by administration of 5 mg of the drug intravenously and then 50, 100, 150, and 200 mg as single oral doses. The rank order for individual steady-state plasma concentrations was the same as that for the relative bioavailability of the 200 mg dose. The area under the plasma concentration-time curve showed a nonlinear increase with the dose. As the relative availability was a good predictor of steady state while clearance after intravenous administration was not, it was concluded that differences in first-pass elimination markedly contribute to the interindividual variability in steady state plasma concentrations. After pentobarbital treatment, the area under the plasma concentration curve of the 200 mg dose was decreased to 32% and 59% of the pretreatment values in two subjects, but there was no change in the plasma half-life of alprenolol. This indicates induction of the first-pass extraction of alprenolol in man.