The partition coefficients between octanol and pH 7.4 buffer for eleven substrates of UDP-glucuronosyltransferase (EC 18.104.22.168) have been determined. They range between 1.1 and 690 in the order p-aminophenol less than phenol less than (o-aminobenzoic acid = o-aminophenol = p-aminobenzoic acid) less than p-nitrophenol less than 4-methylumbelliferone less than mercaptobenzothiazole less than harmol less than phenolphthalein less than 1-naphthol. The effect of Triton X-100, used as a model membrane pertubant, on the enzyme activity of UDP-glucuronosyltransferase in rat liver homogenates towards these substrates was determined and compared with the partition coefficients. Enzyme activities towards p-aminophenol and phenol were decreased by Triton X-100, the enzyme activities towards other acceptor substrates were enhanced maximally with 0.025% (w/v) Triton. "Native" enzyme activity (except for amino containing compounds) and activation could be related to partition coefficient of the substrate. An increase in lipid solubility resulted in reduced enzyme activity in untreated homogenates and greater activation. These results suggest UDP-glucuronosyltransferase lies behind a partially lipid-impenetrable abrrier and it is suggested that this barrier is broken up by membrane perturbants to permit free access of the more lipid-soluble substrates. In addition, the formation in vitro of a glucuronide from mercaptobenzothiazole was demonstrated.