Quantitative structure-activity relationships of the metabolism of drugs by uridine diphosphate (UDP)-glucuronosyltransferase have been investigated. It is observed that most of the variation in the rate of glucuronidation of phenols, thiols, and some other miscellaneous compounds can be accounted for by the lipophilic property of the molecules. The results are consistent with the previous finding with primary and secondary alcohols, and benzoic acids. The optimum lipophilicity for these compounds undergoing metabolism by UDP-glucuronosyltransferase appears to be a log P of 2.