Rat kidney microsomes catalysed the glucuronidation of 1-naphthol, 4-nitrophenol, bilirubin and beta-estradiol. Unlike rat hepatic microsomes, UDP-glucuronosyltransferase activity towards morphine and testosterone was not detectable. Treatment of rats with beta-naphthoflavone resulted in a 3-fold induction of renal UDPGT activity towards 1-naphthol, 4-nitrophenol and phenol, and a 2-fold induction of bilirubin and beta-estradiol glucuronidation. No induction of renal UDPGT was observed after phenobarbital treatment, but renal bilirubin UDPGT activity was specifically induced after treatment of rats with clofibrate. UDPGT activity was purified from rat kidney by a combination of ion-exchange chromatography, gel filtration and affinity chromatography on UDP-hexanolamine Sepharose. One major protein-staining polypeptide was observed on silver-stained SDS-polyacrylamide gels, of molecular weight 55,000 Da, and a minor band of 54,000 Da was also present. Indeed, immunoblot analysis of purified renal UDPGTs with anti-rat liver UDPGT antibodies revealed two immuno-reactive polypeptides of molecular weight 55,000 and 54,000 Da. The highly purified preparations catalysed the glucuronidation of 1-naphthol and bilirubin. Glucuronidation of bilirubin by purified renal UDPGT preparations required the presence of phospholipid, the activity being further enhanced by incubation with rat lung microsomes. The data presented indicate that two UDPGT isoenzymes have been copurified.