Elevations in intracellular Ca2+ ([Ca2+](i)) initiate insulin secretion from pancreatic p-cells, but the secretory responses become rapidly desensitised to maintained elevations in [Ca2+](i). We have investigated the mechanisms underlying the Ca2+ desensitization of insulin secretion using electrically permeabilized rat islets of Langerhans. Measurements of Ca2+/calmodulin-dependent protein kinase II (CaMK II) enzyme activity and immunoreactivity in permeabilized islets demonstrated Ca2+-induced reductions in enzyme activity which could not be attributed to reductions in CaMK II immunoreactive protein. Measurements in intact islets demonstrated that the Ca2+-induced reduction of CaMK II activity was also operative in intact cells, suggesting that this mechanism may have pathophysiological implications for beta-cell function.