The effects of aging on the activities of drug-metabolizing enzymes and antioxidant enzymes were studied in male and female White-Footed mice (Peromyscus leucopus) at ages of 6, 8, 12, 18, 24, 30, 36, and 48 months. Male mice had significantly higher liver microsomal cytochrome P450 (P450) content and NADPH:cytochrome P450 oxidoreductase (P450 reductase) activities than females at all age groups. Many of the P450-dependent enzyme activities were also generally higher in males. Female mice showed age-dependent decreases in P450 content and the activities of P450 reductase, pentoxyresorufin O-dealkylase (PROD) and N-nitrosodimethylamine demethylase (NDMAd) in the liver from 6 to 24 months; while, the males showed an age-dependent decrease only for the liver PROD activity from 6 to 24 months. The old males (30-month old) appeared to have significantly higher activities for 6 beta-, 2 beta-, 16 alpha- and 16 beta-testosterone and androstenedione formation than the middle-aged (6- to 18-month old) and very old (48-month old) males. Females showed age-dependent decreases for the formation of 6 beta-, 2 beta-, 16 alpha- and 16 beta-testosterone in liver microsomes from 6 to 24 months. Lung microsomes from 6- and 8-month old males had much higher activities of ethoxyresorufin O-deethylase (EROD) and PROD than older males. The total NNK alpha-hydroxylation activities changed in the same pattern as lung microsomal EROD and PROD activities in both male and female mice. The activities of several phase II drug-metabolizing enzymes: glutathione S-transferase (GST), DT-diaphorase, sulfotransferase and UDP-glucuronosyl-transferase (UDPGT) did not show any significant age-dependent changes, with the possible exception that the GST activity in males decreased from 18 to 36 months. Males had about 3-fold higher UDPGT activities than females among all age groups. Glutathione peroxidase activities were drastically lower in old and very old males, and 6 to 24 months old males had significantly higher activities than the corresponding females. In females, superoxide dismutase activities decreased linearly to extremely low levels as mice aged. Catalase activities showed a tendency for increase with age in males. In conclusion, some P450-dependent activities and antioxidant enzymes, but not phase II drug-metabolizing enzymes, showed age-dependent changes; and most of these changes occur from 6 to 24 months. The demographic attributes of the White-Footed mouse are well-suited for physiological and biochemical studies of aging and can complement the more standard laboratory mouse model with its typical two year life span.