Pirprofen was well absorbed by man, rhesus monkey, rat, and mouse after oral administration of a solution of 14C-labeled compound. The major route of elimination of radioactivity in all four species was renal, mostly in the form of metabolites. Twelve metabolites of pirprofen, accounting for 80% or more of the urinary radioactivity, were identified in the urine of the four species. The metabolic pathways of pirprofen involved oxidation to the pyrrole analogue, and oxidation of the pyrroline double bond to an epoxide, followed by opening of the oxirane ring to a trans-diol derivative. Scission of the pyrroline or pyrrole ring was also observed, leading to the corresponding aniline-type metabolite, part of which underwent subsequent acetylation. Conjugation of the propionic acid functionality with glucuronic acid was found to be extensive in the mouse, rhesus monkey, and man, but not in the rat. Conjugation with taurine was also observed in the rat and mouse.