Using the pyroantimonate technique, the ultracytochemical distribution of calcium within B cells was studied in isolated rat pancreatic islets incubated during 5, 15 and 30 min with 8.3 mM glucose alone or together with 76 microgram glicazide. Glucose alone produced a continuous increment in the total number of calcium pyroantimonate precipitates (CPP) throughout the incubation period studied. The CPP were mainly associated to the cytoplasmic matrix and the secretory granules at 5 and 15 min and almost evenly distributed between these structures and the plasma membrane at 30 min. Gliclazide plus glucose produced a significant increment, above the glucose values, of the total CPP at 5 min and a later decrease of such values at 15 and 30 min. At 5 min, the incremented total CPP was mainly associated to the secretory granules and the cytoplasmic matrix. The increment in CPP preceded the largest effect of gliclazide on insulin secretion. The latter diminution of CPP induced by gliclazide could contribute to the failure of this drug, as well as other oral hypoglycemic agents, to elicit a second phase of insulin secretion. Changes induced by gliclazide upon B-cell CPP content and distribution might suggest that beyond the effective role of cytosolic calcium in the control of insulin secretion, the cation might reach a threshold concentration in some cell structures to assure the normal development of the secretory process.