Guinea pigs were maintained on four diets which varied only in ascorbic acid (AA), containing 0.3, 1.5, 3.5 and 7.0 mg AA g of diet. The lowest diet contained sufficient AA to prevent scurvy, while the two diets highest in AA content produced liver saturation of the vitamin after 8 weeks. Several hepatic enzymes were measured after 1, 2, 3, 4 and 8 weeks. There were no significant differences among the four groups in the following parameters of microsomal mixed-function oxidation: cytochrome P-450 content, NADPH cytochrome c reductase and aminopyrine N-demethylase. Also, there were no differences in native and nucleotide-activated UDP-glucuronyl transferase activities. However, in the post-microsomal supernatant, overall glutathione S-aryl transferase activity was significantly lower in the group with the lowest supplementation of ascorbic acid, as compared to the three higher groups. Kinetic parameters of aminopyrine N-demethylation were studied in animals maintained for 21 days on a diet deficient in ascorbic acid (< 0.1 mg AA g ), an intermediate control diet (1.5 mg AA g ) and a high-dose AA diet (7.0 mg AA g ). Curvilinear Lineweaver-Burk plots for N-demethylation were obtained in all three groups, and apparent K m and V max calculated separately for both low and high ranges of substrate concentrations. Apparent K m values did not differ significantly among the three groups. Microsomes from AA-deficient animals had V max values lower than the control and high-dose groups. There was no difference in kinetic parameters between microsomes from animals receiving control and high-dose AA diets.