The urine of male and female mice, rats, guinea pigs, rabbits, cats, and dogs, given meperidine hydrochloride, 20--40 mg/kg ip, was analyzed by GLC for meperidine, normeperidine, p-hydroxymeperidine, and total (free and conjugated) meperidinic and normeperidinic acids. More than 90% of the excreted drugs was found in the 24-hr urine. Meperidine was observed in the urine of mice, rats, guinea pigs, and cats, but only a trace amount was observed in the urine of rabbits and dogs. Normeperidine, p-hydroxymeperidine (except in the mice), and total meperidinic and normeperidinic acids were observed in all species. All of the species studied have the capacity to N-demethylate meperidine to normeperidine and to hydrolyze meperidine and normeperidine to their respective acids. The male has a higher N-demethylating activity that the female with the exception of mice. Ester hydrolysis is a major metabolic pathway for meperidine metabolism.