Recently, we found that mitragynine, a major constituent of Mitragyna speciosa, has an opioid agonistic activity, but its weak potency could not explain the opium-like effect of this plant. In the present study, bioassay-guided fractionation of the crude extract of the leaves of M. speciosa was carried out to search for potent opioid agonists other than mitragynine. Opioid agonistic activities were evaluated using twitch contraction induced by electrical stimulation in guinea-pig ileum. The crude extract of M. speciosa inhibited the twitch contraction in a concentration-dependent manner. The inhibition was reversed by naloxone. The opioid effect was detected only in the crude base fraction, which was followed by the isolation of five indole alkaloids. Among these alkaloids, 7-hydroxymitragynine showed the most potent opioid effect on the electrically-stimulated contraction (pD (2) = 8.38 +/- 0.12). The potency, calculated using pD (2) values, was 30- and 17-fold higher than that of mitragynine and morphine, respectively. Antagonism of naloxone on concentration-response curves for 7-hydroxymitragynine confirmed its opioid effect. These results suggest that the opioid effect of M. speciosa is mostly based on the activity of 7-hydroxymitragynine.