In vivo stimulation of the exocrine pancreas with concentrations of secretagogue in excess of a maximally stimulating dose causes a marked disturbance of the intracellular segregation, transport, and exocytosis of digestive enzyme zymogens. Under physiological conditions elements of the cytoskeleton, most notably microtubules and microfilaments, are involved in the regulation of these intracellular events. We infused caerulein, a peptide analogue of cholecystokinin, at a supramaximal dose (10 micrograms.kg-1.h-1 for up to 6 h) intravenously in rats. To study the ultrastructural alterations of acinar cell microfilaments and microtubules by immunogold labeling, we used monoclonal antibodies directed against actin and beta-tubulin. As early as 30 min after the start of the secretagogue infusion we found a progressive disassembly of microtubules and microfilaments in exocrine cells. In immunoblot studies this disassembly of the cytoskeleton was paralleled by a degradation of its structural proteins actin and beta-tubulin. Our results indicate that the earliest morphological events during supramaximal secretagogue stimulation of the pancreas involve the disassembly and degradation of microtubules and microfilaments. This cell biological phenomenon offers an explanation for the disturbances of segregation, transport, and exocytosis of digestive enzymes, which are known to be associated with supramaximal stimulation of the pancreas and experimental models of pancreatitis.