OBJECTIVES: The effect of aging on drug metabolism in humans has not yet been completely described. METHODS: Two hundred twenty-six patients with equal histopathologic conditions were investigated. The cytochrome P450 contents in the liver biopsy samples, the plasma antipyrine clearance rates after oral administration and, as an independent control of vitality, serum testosterone levels were determined. RESULTS: Cytochrome P450 content in subjects from 20 to 29 years of age was 7.2 +/- 2.6 nmol.gm-1, increased during the fourth decade (+7.2%, p = NS), declined after 40 years (-16%, p