In both acute and chronic liver disease in man, elimination of drugs metabolized by the cytochrome P450 (CYP) enzymes is impaired. In contrast, those drugs metabolized by UDP-glucuronosyltransferase (UGT) have a relatively normal elimination. Studies in rats with experimentally induced liver injury also show this relative preservation of glucuronidation. In liver disease, a number of factors, including inflammation, fibrosis and regeneration, may be associated with this differential effect on drug metabolism. Partial hepatectomy provides a model in which to isolate the effects of liver regeneration on drug metabolism. Partial hepatectomy or sham operation was performed in 24 male Sprague-Dawley rats and three rats from each group were studied at days 1, 2, 4 and 6. Comparison between CYP and UGT was made at the protein level using immunohistochemistry and immunoblotting probed with a polyclonal antibody to UGT, identifying both family 1 and family 2 isoforms, and an antibody to the CYP isoform CYP2C11. Steady state messenger RNA levels of four isoforms of UGT were assessed by northern blot analysis. By both immunohistochemistry and immunoblotting, the level of CYP protein decreased from day 2 to 6 after hepatectomy. In contrast, the UGT protein level was not altered by partial hepatectomy. Northern blot analysis of UGT isoforms demonstrated differential regulation of isoforms from the two major families. The UGT family 1 isoforms were initially markedly depressed following partial hepatectomy and then steadily rose over 6 days to greater than the level in controls. In contrast, there was an apparent increase in UGT2B1 mRNA (not significant) on day 2, while UGT2B3 mRNA was maintained over the six days. These results demonstrate that during hepatic regeneration the protein content of total UGT is normal, while CYP2C11 protein is markedly reduced. Northern blot analysis suggests that individual isoforms of UGT are differentially regulated during the regeneration process.