The purpose of this study was to determine what histological changes, if any, accompany liver enlargement and microsomal enzyme induction in rats administered high doses of therapeutic agents in preclinical toxicology studies. This was accomplished by evaluating a database derived from a series of 11 induction studies in rats with 10 novel compounds comprising five therapeutic classes. Results from serum enzyme chemistry analyses, gross organ weight changes, and histological analyses of the liver sections were evaluated and compared with the magnitude and extent of hepatic cytochrome P450 induction. All compounds were administrated via oral intubation once a day for the duration of the study using multiple doses, each proportionally based on body weight. During the course of these studies, serum clinical chemistry data and clinical observations were recorded. After necropsy, histopathology observations were made, and hepatic microsomes were assayed for cytochrome P450 content and associated drug-metabolizing enzymes. In some cases, cyanide-insensitive beta-oxidation of palmitoyl CoA was also assayed. Liver weight increases of 20% or greater were associated with histological evidence of hypertrophy, but neither the severity of hypertrophy nor the magnitude of liver weight increase correlated with the magnitude of drug-metabolizing enzyme elevations. Hypertrophy alone was not associated with serum enzyme increases. While there was a correlation between the incidence of increased liver weights and microsomal enzyme induction, the magnitudes of these increases were not related. Decreased serum triglycerides were often associated with elevated beta-oxidation attributed to hepatic peroxisome proliferation. It was concluded that, while slight ALT elevations occasionally were observed, hepatic microsomal enzyme induction was generally not accompanied by substantial morphological changes or elevated serum enzyme levels considered indicative of liver injury.