Glucuronidation is a major route of clearance for a diverse set of both drug and endogenous substrates. The present study was undertaken to redress the lack of molecular information currently available on drug glucuronidation by the dog, a species widely used in metabolism studies by the pharmaceutical industry. A novel dog uridine diphosphate glucuronosyltransferase (UGT), designated UGT2B31 (GenBank Accession Number: AY135176), has been isolated from a dog cDNA library, expressed in V79 cells and characterised using various methods: (i) UGT2B31 sequence has been compared with mammalian UGT sequences using both sequence alignments and phylogenetic analysis; and (ii) the substrate specificity of UGT2B31 has been determined using functional analysis and compared with that obtained using UGT2B7 and dog liver microsomes. The following results were obtained: (i) sequence alignments between UGT2B31 and UGT2B15 gave the greatest degree of identity (76%); however, human UGT2B4, human UGT2B7, monkey UGT2B9 (all 75%), and rat UGT2B1 (73%) also gave a high degree of identity; (ii) phylogenetic analysis determined UGT2B31 to be most closely related to rat UGT2B1; (iii) UGT2B31 displayed a substrate specificity similar to human UGT2B7 and rat UGT2B1, catalysing the glucuronidation of phenols, opioids, and carboxylic acid-containing drugs; and (iv) UGT2B31 only formed morphine-3-glucuronide; however, kinetic analysis determined the K(m) of this reaction to be similar to that observed with UGT2B7 (both approximately 1300 microM). The results suggest that UGT2B31 plays a crucial role in drug detoxification by the dog and may be the canine equivalent of human UGT2B7.