The development of gap junctions between pancreatic B-cells was quantitatively assessed in freeze-fracture replicas of isolated rat islets under different conditions of insulin secretion. The results show that in resting B-cells, gap junctions are small and scarce but that these junctions increase when insulin secretion is stimulated. Both a short (90 min) stimulation by glucose in vitro and a prolonged (2.5 d) stimulation by glibenclamide in vivo raise the number of gap junctions; in addition, the glibenclamide stimulation causes an increase in the size of individual gap junctions. As a consequence, the total area occupied by gap junctions on the B-cell membrane and the ratio of this area to the cell volume were found significantly increased in the latter condition. The slight increase of these values observed after the glucose stimulation did not reach significance. These data indicate a change of gap junctions during the secretory activity of the pancreatic B-cells. The possibility that the coupling of the cells is affected by the treatment is discussed.