1. The psychoactive cannabinoids (-)-delta 9-tetrahydrocannabinol ((-)-delta 9-THC) and the 1,1-dimethyl-heptyl homologue of (-)-11-hydroxy-delta 8-tetrahydrocannabinol ((-)-DMH) both inhibited electrically-evoked contractions of the mouse isolated vas deferens and the myenteric plexus-longitudinal muscle preparation of the guinea-pig small intestine. 2. Concentrations of (-)-delta 9-THC and (-)-DMH that decreased twitch heights by 50% were 6.3 and 0.15 nM respectively in the mouse vas deferens and 60 nM and 1.4 nM respectively in the myenteric plexus preparation. (-)-DMH was about 40 times more potent than (-)-delta 9-THC in both preparations, supporting the notion that their mode of action in each tissue is the same. 3. The psychically inactive cannabinoid, (+)-DMH, had no inhibitory effect in the mouse vas deferens at a concentration of 30 nM, showing it to be at least 1000 times less potent than (-)-DMH. In the myenteric plexus preparation, (+)-DMH was about 500 times less potent than its (-)-enantiomer. 4. The inhibitory effects of sub-maximal concentrations of (-)-delta 9-THC were not attenuated by 300 nM naloxone. 5. The findings that (-)-delta 9-THC and (-)-DMH are highly potent as inhibitors of the twitch response of the mouse vas deferens and guinea-pig myenteric plexus preparation and that DMH shows considerable stereoselectivity suggest that the inhibitory effects of cannabinoids in these preparations are mediated by cannabinoid receptors.