A morphometrical investigation was carried out on rat pancreatic lobules, which were incubated for 10 min in MEM. Some lobules were treated with carbamylcholine chloride (CCh) at three different concentrations; those not treated were defined as controls. Upon stimulation, the apical plasma membrane surface area and the number of zymogen granules less than 20 nm away from the apical plasma membrane significantly increased and were directly correlated with increases in secretion. The diameter of zymogen granules decreased at 10(-6) M CCh concentration, but increased at 10(-7) and 10(-5) M CCh concentrations as compared with controls. These results seem to indicate that exocytosis, rather than transmembrane flow of individual molecules, is enhanced by and correlated with protein secretion from the exocrine pancreas.