5-Hydroxytryptamine and substituted benzamides such as cisapride and BRL 24924 enhance the twitch responses of the electrically stimulated longitudinal muscle-myenteric plexus preparation of the guinea-pig. The effects of these benzamides and 5-HT could possibly be mediated via similar receptor-effector systems. The aim of our study was therefore to determine whether R 50 595, an analogue of cisapride devoid of intrinsic activity, could specifically interfere with the effects of cisapride and BRL 24924 and if so, whether it would also affect the responses to serotonin. R 50 595 had no effect on the twitch responses of the electrically stimulated preparation up to a concentration of 3 X 10(-7) M. Cisapride and BRL 24924 both enhanced the contractile response to electrical stimulation by a maximum of 37 +/- 7% at 3 X 10(-7) M for cisapride and 36 +/- 6% for BRL 24924, also at 3 X 10(-7) M. R 50 595 (10(-7)(-3) X 10(-7) M) antagonized the effects of cisapride and BRL 24924 in a non-competitive way. 5-HT enhanced the contractile responses by a maximum of 24 +/- 3.2% at 3 X 10(-8) M. The effects of 5-HT were completely abolished at a concentration of 3 X 10(-7) M R 50 595. R 50 595 also antagonized the effects of 5-HT in a non-competitive way.(ABSTRACT TRUNCATED AT 250 WORDS)