Metabolism of an antipyretic and analgesic of aniline derivative, bucetin (β-hydroxybutyro-p-phenetidide, Ia) in rabbits was studied. From the results of estimations of total glucuronic acid and sulfate in the urine, it was deduced that the majority of metabolites was excreted as glucuronides and the excretion of sufates was negligiblly small, if any. By treating the urine with β-glucuronidase, extracting with ether, and purifying the extract through silica gel column, N-(β-hydroxybutyro)-p-aminophenol (Ib) and phydroxyacetanilide (IIIb) were isolated and identified with authentic samples. Moreover, p-hydroxyacetoacetanilide (IIb) was identified in the ether extract by thin-layer chromatography. For the formation of IIIb, there were two reactions involved ; de-ethylation of Ia at ethoxyl group and the loss of two carbons in β-hydroxybutyryl group. Since the latter reaction was not familiar for drug metabolism, the mechanism of the reaction was also discussed in this communication.