It has been reported that morphine-6-glucuronide inhibits small intestinal transit in mice more potently than morphine. In this study, we investigated the effects of morphine, morphine-6-glucuronide and morphine-3-glucuronide on the contractile response of the circular muscle isolated from mouse ileum. Morphine and morphine-6-glucuronide induced tonic contraction dose-dependently, and the contractile force of morphine-6-glucuronide was greater than that of morphine. Morphine-3-glucuronide induced slight contraction at high dose. These results suggest that the strong contraction induced by morphine-6-glucuronide contributed to the inhibition of small intestinal transit in mice.