The expression of specific UDP-glucuronosyltransferase isoforms in 2-acetylaminofluorane—induced rat liver preneoplastic nodules was studied; livers from pair-fed littermates were used as controls. For comparison, liver and kidney from 3-methylcholanthrene—treated or untreated (control) rats were used. Steadystate UDP-glucuronosyltransferase mRNA levels were determined by Northern blot analysis or in situ hybridization of tissue sections using a 30-mer oligonucleotide specific for the 3-methylcholanthrene—inducible UDP-glucuronosyltransferase (which is active toward 4-nitrophenol) or a double-stranded cDNA probe specific for androsterone—UDP-glucuronosyltransferase. For 3-methylcholanthrene—inducible UDP-glucurono-syltransferase, the mRNA level was very low in control liver; there was a 15-fold increase after 3-methylcholanthrene treatment. This mRNA was present at relatively high concentration in the kidney and there was a threefold increase after 3-methylcholanthrene administration. In livers with preneoplastic nodules 1 mo after cessation of carcinogen administration, this mRNA concentration was approximately 15 times greater than in control liver. Similar changes in the level of the 3-methylcholanthrene—inducible UDP-glucuronosyltransferase were also observed by in situ hybridization of tissue sections. Immunocytochemical studies using an antiserum that recognizes the 3-methylcholanthrene–inducible UDP-glucuronosyltransferase showed a marked increase in the concentration of this isoform in preneoplastic nodules compared with the adjacent nonnodular liver. Consistent with the changes in mRNA levels, there was a threefold increase in 4-nitrophenol–UDP-glucuronosyltransferase activity in 3-methylcholanthrene–treated rat livers and in livers with preneoplastic nodules. In contrast to the 3-methyl-cholanthrene–inducible UDP-glucuronosyltransferase mRNA, the androsterone–UDP-glucuronosyltransferase mRNA was abundant in the liver and nearly absent in the kidney; its concentration was not increased after 3-methylcholanthrene administration or in preneoplastic nodules. Immunocytochemical studies using an antiserum that recognizes androsterone- testosterone– and bilirubin–UDP-glucuronosyltransferase isoforms did not show any increase of the concentration of these antigens in preneoplastic nodules. Similarly, activities for androsterone, testosterone and bilirubin also did not increase after 3-methylcholanthrene administration or in livers bearing the preneoplastic nodules. (HEPATOLOGY 1991;13:38—46).