The glucuronidation capacity of rat brain microsomes was investigated using a series of chemically related phenolic compounds and fatty acids which are usually glucuronidated in vivo. Most of the phenols assayed were glucuronidated, but no glucuronide formation was detected for stearic and alpha-linolenic acids, 4-methylphenol, bilirubin, morphine, dopamine and serotonin. The activity of uridine diphosphate-glucuronosyltransferase (UGT) towards 1-naphthol represented 0.28% of that obtained with liver microsomes. The inhibitory effects on the formation of 1-naphthol glucuronide of some endogenous and exogenous substances were investigated. The results suggest that only the isoform of UGT conjugating 1-naphthol is present in rat brain.