The effects of antipyrine (1200 mg day-1), phenobarbitone (100 mg day-1) and rifampicin (600 mg and 1200 mg day-1, respectively) administration for 7 days on sparteine metabolism and 6 beta-hydroxycortisol excretion were studied in panels of extensive (EM) and poor metaboliser (PM) subjects. Drug metabolism was induced in both EM and PM subjects by antipyrine and rifampicin pretreatment as indicated by increased excretion of 6 beta-hydroxycortisol. A 30% increase in metabolic clearance of sparteine was observed in EM subjects following rifampicin administration whereas in PM subjects no effect on the overall elimination of the drug was seen. The data indicate that the regulation of cytochrome P-450 isozyme involved in polymorphic debrisoquine/sparteine metabolism is predominantly under genetic control and that enzyme induction exerts only a marginal effect.