This investigation was carried out (a) to determine if the enzyme inductive effect produced by phenobarbital reduces the interindividual variability in the biotransformation of a drug, as suggested in the literature; (b) to test whether the intrinsic clearance of free drug, for drugs exhibiting restrictive clearance, reflects the activity of drug-metabolizing enzyme systems; and (c) to determine if enzyme induction affects the apparent volume of distribution of a drug that tends to concentrate in the liver. Twelve pairs of adult male rats, matched with respect to their serum warfarin free fraction, received an intravenous injection of (S)-(-)-warfarin, 0.6 mg/kg, after four daily injections of either saline solution or phenobarbital (75 mg/kg). Phenobarbital treatment increased both the total and intrinsic clearance of (S)-(-)-warfarin almost threefold but did not reduce the coefficient of variation of the intrinsic clearance. Serum protein binding of (S)-(-)-warfarin was not affected by phenobarbital treatment. The biological half-life of warfarin and the duration of its anticoagulant effect were reduced substantially by treatment with phenobarbital. Consistent with pharmacokinetic theory, the relationship between total clearance and the free fraction of warfarin in serum remained approximately linear, but the slope of the regression line was increased for the animals treated with phenobarbital.