We previously demonstrated that the supramaximally effective concentrations of caerulein caused marked changes in the apical cytoskeleton of the rat pancreatic acinar cell. These changes included ablation of microvilli, the terminal actin web, and intermediate filament bands. The present study was designed to elucidate part of the intracellular signalling mechanism mediating these changes. For these studies we used a cholecystokinin (CCK) analogue, CCK-IMV-180, that has been previously demonstrated not to inhibit enzyme secretion and to prevent the inhibition caused by caerulein. We investigated the effects of CCK-JMV-180 alone and in combination with supra maximal concentrations of caerulein on the morphology of the apical structures, on 1, 2-diacylglycerol production (a measure of phospholipase C activity), and on amylase secretion in rat pancreatic acini. Supramaximally effective concentrations of caerulein caused inhibition of enzyme secretion. CCK-JMV-180 had no effect on the ultrastructure of the apical region of the acinar cell and it prevented the ablation of apical cytoskeleton induced by a supramaximal concentration of caerulein (10 nM). CCK-JMV-180 inhibited the increase in 1, 2-diacylglycerol formation and the inhibition of amylase release caused by 10 nM caerulein. Mimicking the effect of 1, 2-diacylglycerol on activation of protein kinase C with phorbol 12-myristate 13-acetate and reproducing changes in [Ca 2+] i caused by 10 nM caerulein with 100 nM bombesin did not alter the apical cytoskeleton. These results suggest that the cytoskeletal changes observed with inhibitory concentrations of caerulein are caused by the phospholipase C effects of caerulein on membrane phospholipids.