Interferon down-regulates expression of cytochrome P-450 3A in male rats. This study explored the hypothesis that interferon therefore decreases the metabolism of drugs catalyzed by cytochrome P-450 3A. Initial experiments in male rats used microsomal erythromycin N-demethylase activity as a probe for cytochrome P-450 3A catalytic activity. After administration of rat interferon-y, erythromycin metabolism was impaired (53% of control; p < 0.01). This change correlated with the decline in cytochrome P-450 3Adependent androstenedione 6p-hydroxylase activity, indicating that the decrease in erythromycin N-demethylase activity could be attributed to interferon-mediated suppression of cytochrome P-450 3A. We then used the [l*C]N-methyl erythromycin breath test to assess the activity of hepatic cytochrome P-450 3A in rats and human subjects before and after a single dose of interferon. In rats, rat interferon-y decreased erythromycin metabolism to 57% of control (p < 0.005). In the human study, six patients with chronic active hepatitis C and four healthy controls were examined 20 to 26 hr after receiving a subcutaneous injection of human interferon-%,. Interferon produced a small decrease (median = 15%; range = 3% to 35%) in erythromycin metabolism (p < 0.05), as determined by 2-hr excretion of 14C0, in the breath. Thus interferon-mediated suppression of cytochrome P-450 3A is less strong in human subjects than in male rats. Comparison of our data about the effect of interferon on cytochrome P-450 3A activity with earlier observations of a major impairment of theophylline clearance in human beings suggests that in human subjects interferon may have a more important effect on other drugmetabolizing enzymes, such as those of the cytochrome P-450 1A subfamily. However, interferon may produce clinically relevant impairment in the elimination of drugs that are substrates for cytochrome P-450 3A, should such agents have a narrow therapeutic index. (HEPATOLOGY 1993; 17:230-235.)