Abstract.Sulfonylureas such as BS4231 and HB419 stimulate insulin secretion in pieces of rat pancreatic tissue. Their insulinotropic effect requires the presence of a sufficient amount of calcium in the incubation medium. Both sulfonylureas markedly stimulate calcium uptake by isolated islets of Langerhans, their stimulant action being more marked in the absence of glucose or at low glucose concentration than at higher glucose levels. Theophylline alone does not stimulate insulin secretion, but it significantly enhances insulin release in the presence of BS4231. This finding is interpreted as another indication that sulfonylureas, like glucose or leucine, favour calcium accumulation in the beta cell by modifying the balance between calcium influx and efflux across the plasma membrane. Heavy water, a microtubules‐stabilizer, inhibits the insulinotropic action of sulfonylureas, but does not affect BS4231‐induced calcium uptake. These data suggest that the insulinotropic action of sulfonylureas results from the accumulation of calcium in the beta cell, and the subsquent activation of the microtubular‐microfilamentous system involved in the emiocytosis of insulin secretory gra