Acyl glucuronides have been shown to be reactive electrophilic metabolites capable of undergoing hydrolysis, rearrangement (isomerization via acyl migration) and covalent binding reactions to plasma protein. The present study was undertaken to explore the occurrence and extent of in vivo formation of covalent adducts of diflunisal (DF), a salicylate derivative which forms a reactive acyl glucuronide, with tissues and plasma protein of rats. Groups of rats were given 50 mg DF/kg i.v. twice daily for periods of up to 7 days. Steady state plasma concentrations of reversibly bound DF and its conjugates (as measured 6 hr after a dose) were achieved by the third day of dosing. T 1/2 values after cessation of dosing were about 5-10 hr. By contrast, covalent DF-tissue adducts steadily accumulated over the 7-day dosing period. Maximum concentrations, measured 6 hr after the last dose, were 4.8 (liver), 1.0 (kidney), 0.74 (plasma), 0.26 (small intestine minus contents), 0.27 (large intestine minus contents) and 0.20 (skeletal muscle) microgram DF/g tissue or/mL plasma. T 1/2 values of about 50, 67, 18, 38 and 43 hr were obtained for liver, kidney, plasma and small and large intestine (respectively) after cessation of dosing. Thus, the study of acyl glucuronide reactivity and the question of any derived toxicity or immune responses should consider the formation of long-lived adducts in tissues as well as in plasma.