Small and big mouse islets were compared with special reference to their content of glucagon-producing alpha-cells and somatostatin-producing delta-cells. Areas stained for glucagon and somatostatin were measured in the largest cross section of small (diameter < 60 mu m) and big (diameter > 100 mu m) islets. Comparison of the areas indicated proportionally more delta- than alpha-cells in the small islets. After isolation with collagenase these islets were practically devoid of alpha-cells. We evaluated the functional importance of the islet size by measuring the Ca2+ signal for insulin release. A majority of the small islets responded to the hyperpolarization action of somatostatin with periodic decrease of cytoplasmic Ca2+ when glucose was elevated after tolbutamide blockade of the K-ATP channels.