An animal model suitable for in vivo studies of interferon-mediated suppression of hepatic oxidative drug metabolism has been developed. Rats were injected with either recombinant human interferon alpha A, recombinant human interferon gamma, recombinant rat interferon gamma, or vehicle and experiments were performed 24 h later. In some animals theophylline elimination was determined twice (10 days apart), once after interferon and once after vehicle. Theophylline clearance was also determined in the isolated perfused rat liver after pretreatment of animals with interferon or vehicle. Pretreatment of animals with rat interferon gamma significantly reduced theophylline clearance in the intact rat but neither human interferon alpha A nor human interferon gamma altered theophylline elimination in vivo. Similar results were observed in the isolated perfused rat liver. We then examined whether the effects of interferon on hepatic drug metabolism were generalized or confined to individual cytochrome P450 isozymes; androstenedione hydroxylation pathways were used as catalytic probes for individual cytochrome P450 isozymes. Rat interferon gamma (but not human interferon alpha A) decreased levels of total hepatic microsomal P450 and reduced androstenedione 16 beta-hydroxylation. The formation of three other hydroxylated androstenedione metabolites appeared reduced to a similar extent, although these changes were not significant. It is concluded that autologous but not heterologous interferons impair oxidative drug metabolism in the rat. The reduction of hepatic P450 produced by interferon may result from the suppression of multiple isozymes.