In this study, trimethadione (TMO) was chosen as a model drug to investigate the extent of hepatic drug-oxidizing capacity. It was also studied whether pretreatment of rats with ibuprofen selectively affected the formation of antipyrine metabolites. The ratios of serum dimethadione (DMO)/TMO and the urinary metabolite formation of antipyrine in the rat pretreated with ibuprofen 50 mg/kg daily for 7 days were not changed as compared to controls. These results indicate that the short-term administration of ibuprofen did not impair TMO metabolism and changed antipyrine and its metabolite formation in the rat 24 h urine.