The intracellular injection of 6-carboxyfluorescein in endocrine cells of cultured Rat pancreas followed by the characterization, by electron microscopy, of the injected cells shows that the dye is transferred from the injected cell into adjacent cells, presumably by gap junctions (nexus)= intercellular coupling. The transfer occurs not only between insulin-containing cells (B-cells), but also between B-cells and glucagon containing cells (A-cells) or somatostatin-containing cells (D-cells). The demonstration of homo-and heterocellular coupling in the endocrine pancreas supports the hypothesis that a direct exchange of molecules between islet cells may participate in the regulation of the secretory behavior of the islet of Langerhans.