Cholecystokinin (CCK) and acetylcholine, at concentrations greater than those required for maximal pancreatic enzyme secretion, elicit a submaximal secretory response. The mechanism for this "secretagogue-induced unresponsiveness" is unknown. Using isolated pancreatic acini of the mouse, we now find that high concentrations of secretagogues also induce a profound alteration in acinar morphology, characterized by the formation of spherical protrusions on the basal surface of the cells. Since both the determination of cell shape and exocytosis may involve calcium and contractile proteins, we used a calcium-free medium and cytochalasin B (CB) to evaluate the importance of a contractile mechanism in the secretory and morphological effects of high concentrations of CCK-octapeptide (CCK8). Incubation in a calcium-free medium partially blocked CCK-induced unresponsiveness, but brought about dissociation of the acini. CB at a concentration of 3 micrograms/ml caused the disappearance of apical microfilaments and, most strikingly, completely prevented the morphological alteration induced by CCK8. Furthermore, CB converted the biphasic dose-response curve for CCK8-induced amylase release to a monophasic shape, such that the amylase release stimulated by a high concentration of CCK8 (10 nM) was augmented. It is concluded, therefore, that a contractile process involving microfilaments may mediate "secretagogue-induced unresponsiveness" in pancreatic acinar cells.