The numerical and spatial distribution of gap junctions between insulin-containing cells (beta-cells) under resting and stimulated conditions of insulin secretion were quantitatively analyzed in freeze-fracture replicas of isolated rat islets of Langerhans. The results show that the beta-cells located at the periphery of the islet have twice as many gap junctions per unit membrane area as the beta-cells situated in the islet center. In both locations, gap junctions assumed a nonrandom clustering on the beta-cell membranes. During stimulation of insulin secretion, the gap junctions were found increased between the central and between the peripheral beta-cells. The degree of their clustering was also modified. The latter change depended both on the location of the gap junctions in the islet and on the type of stimulation used (high glucose or glibenclamide).