In rats fed a diet supplemented with N-2-fluorenylacetamide or N,N-dimethyl-p-phenylazoaniline, the activity of a number of microsomal and soluble liver enzymes was observed over a 24-week period. During this time the activity of the following enzymatic reactions was estimated periodically: glucuronyl transferase, the N-demethylation of morphine, the hydroxylation of acetanilide, tryptophan peroxidase, glutathione reductase, uridinediphosphateglucose dehydrogenase, and the reduction of the 4,5 double bond of ring A of cortisone. The activity of the enzymes, which N-demethylate morphine and hydroxylate acetanilide, decreased progressively toward the time when hepatoma formation occurs. In contrast, the glucuronyl transferase and the glutathione reductase showed significant increases in activity. N-2-fluorenylacetamide also induced a marked rise in the activity of the uridinediphosphateglucose dehydrogenase.