The peptide caerulein (CK) is a potent muscle stimulant of guinea pig ileum and its effect is Ca2+-dependent. Ca2+ antagonist drugs administered to the organ perfusion solution diminished the twitches of the electrically stimulated ileum. Subsequent administration of caerulein reinforced this inhibition of contraction and acetylcholine (ACh) release by the ileum. This effect was antagonized by cholecystokinin antagonists (i.e., proglumide and loxiglumide) and by dibutyryl-cyclic-guanosine monophosphate, which overcame the stimulant effect of the peptide. These findings indicated that in the guinea pig ileum, caerulein exerts two opposite effects, both of which are mediated by the same receptor: a stimulation effect, with an increase in acetylcholine release when the organ is incubated in normal saline, and a relaxation effect, with a reduction in acetylcholine release when the ileum is bathed with calcium antagonist drugs.