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. Steady-state 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-glucuronosyltransferase, 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.