Species difference in glucuronidation of morphine was studied using mice, rats, guinea pigs and rabbits in vivo and in vitro. Morphine-3-glucuronide (M-3-G) and morphine-6-glucuronide (M-6-G) were determined by high-performance liquid chromatography. M-3-G was the major urinary metabolite of morphine in all these animal species. However, a remarkable species difference was observed in the urinary excretion of the M-6-G. Excretion ratios of the M-3-G to M-6-G were approximately 4 : 1 and 50 : 1 in guinea pigs and rabbits, respectively. The urinary excretion of M-6-G in mice and rats was too small to be determined. On the other hand, the ratios of uridine diphosphateglucuronyltransferase (UDPGT) activities toward 3-and 6-hydroxyl groups of morphine in liver microsomes of mice, rats, guinea pigs and rabbits were approximately 300 : 1, 90 : 1, 4 : 1 and 40 : 1, respectively. Ratios of two morphine UDPGT activities in the liver microsomes of guinea pigs and rabbits, thus, reflected those of urinary excretion of morphine glucuronides.