The mechanism of action of B-HT 933 (azepexole) was studied on the rat vas deferens and myenteric plexus-longitudinal muscle (MP-LM) of the guinea-pig ileum. The drug caused a concentration-dependent inhibition of the twitch response in both preparations. The maximal inhibition in both preparations was 80-90%. B-HT 933 did not affect the cumulative dose-response curves of the vas deferens and of MP-LM to noradrenaline (NA) and acetylcholine (Ach) respectively. Yohimbine antagonized in a competitive way the twitch inhibitory effect of B-HT 933 on vas deferens and on MP-LM; the pA2 values were 8.62 and 8.5, respectively. The twitch inhibitory effects of B-HT 933 were not antagonized by propranolol. The results suggest that the action of B-HT 933 is mediated by stimulation of presynaptic alpha 2-receptors.