AbstractChanges in hippocampal calbindin immunoreactivity were investigated after repeated electroconvulsive shocks. Adult rats were subjected to 10, 20, or 36 electroconvulsive shocks (50 mA, 0.5 seconds), given as on shock per day, 5 days a week. The rats were sacrificed and processed for calbindin immunohistochemistry 1, 2, and 30 days after the last electroconvulsive shock. In the rats receiving 10 or 20 electroconvulsive shocks, a selective reduction of the calbindin immunoreactivity of the dentate granule cell bodies, dendrites, and mossy fibers was noted 1 and 2 days after the last electroconvulsive shock. After 36 electroconvulsive shocks there was an almost complete loss of calbindin immunoreactivity from the granule cell bodies and dendrites, and the calbindin immunoreactivity of the mossy fibers was markedly reduced. Thirty days after the last of 36 electroconvulsive shocks, the calbindin immunoreactivity was back to normal. Besides demonstrating pronounced changes associated with repeated electroconvulsive shocks, the results confirm the transient nature of these changes.