The contractile effect of the dihydropyridine analogue Bay K 8644, recently described as a Ca2+-agonist, has been evaluated in the rat isolated duodenal muscle in comparison with that of acetylcholine. Bay K 8644 (3 X 10(-10)-10(-6) mol/l) increased the duodenal motility, whereas it behaved as an inhibitor when tested at high concentrations (greater than 10(-6) mol/l). Bay K 8644 was more potent than acetylcholine (threshold concentration 3 X 10(-10) vs. 10(-8) mol/l) but less efficient. The contractile responses to Bay K 8644 and acetylcholine were reduced in calcium-free medium and abolished by addition of 1 mmol/l EGTA. In some K+-depolarized tissues, however, a contractile response to Bay K 8644 was still observed. The effects of Bay K 8644 and acetylcholine were antagonized by nifedipine and verapamil; however, a competitive antagonism was observed only for the interaction Bay K 8644-nifedipine (pA2 = 9.86). The contraction induced by Bay K 8644 was noncompetitively antagonized by low concentrations of atropine (10(-7) mol/l); Bay K 8644 (10(-7) mol/l) did not modify the response to acetylcholine, while inhibiting it at higher concentration (10(-5) mol/l); in addition, it was able to enhance the response to endogenous acetylcholine, released by field stimulation. The above results indicated that Bay K 8644 and cholinergic stimuli are strong activators of the duodenal motility in vitro. The interference of Bay K 8644 with the cholinergic system, observed in both unstimulated and electrically stimulated strips, suggested a novel site of action of this compound in the gastrointestinal muscle.