Modulation of serotonin (5-HT)(3)-receptor function by repeated electroconvulsive shock (ECS) treatment was investigated to elucidate the mechanism underlying the effectiveness of electroconvulsive therapy (ECT) which is clinically used for drug-resistant depression. ECS (100 V, 1 s) was applied once a day for 14 days via an electrode placed on the ears of rats (ECS group). For controls, rats were handled similar to the ECS-treated group except for the stimulation. Hippocampal slices (thickness, 450 microm) were prepared 24 h after the final ECS treatment when rats were 5-6 weeks old. Intracellular recordings were made from the neurons in the CA1 pyramidal cell layer. Activation of 5-HT(3) receptors increased spontaneous postsynaptic potentials (sPSP). Increased sPSP was mainly mediated by GABA. The 5-HT(3) receptor mediated increase in sPSP was potentiated in the ECS group compared with the control group. These findings suggest that the 5-HT(3) receptor function is potentiated by repeated ECS.