The acute ultrastructural changes induced by glucose upon the B cells were studied in normal rat pancreas perfused with 3.3 and 16.6 mmol/l glucose. A significant increment in the volume of the RER, microtubule, mitochondria, lysosomes and B granules was induced by 16.6 mmol/l glucose, while no significant changes were detected in the total B cell volume or in the size of the nucleus, cytoplasm and Golgi complex. The number of secretory granules was greatly reduced in B cells obtained from pancreas perfused with 16.6 mmol/l glucose, while its diameter was significantly enhanced. In these cells both, the number of pale granules as well as those attached to the cell membrane, were increased. All these data suggest that the increase in the extracellular glucose concentration produces not only the classical biphasic secretion on insulin, but also induces significant and measurable changes in the volume of several B cell organelles. Such ultrastructural changes correlate well with the well-known effect of glucose upon the metabolism of these cells.