Effects of repeated electroconvulsive shock (ECS) treatment on 5-hydroxytryptamine (5-HT) response were investigated to elucidate the ECS-induced changes, which may be related to antidepressant effects, using electrophysiological methods with hippocampal slices in vitro. ECS was applied to Wistar rats once daily for 14 d from 3 wk of age (ECS group). Control animals did not receive ECS (control group). Twenty-four hours after the final ECS treatment, hippocampal slices were prepared for intracellular recording analysis. Application of 5-HT (0.1-30 µm) caused a dose-dependent hyperpolarization in hippocampal CA1 neurons. 5-HT-induced hyperpolarization in the ECS group was significantly greater than that in the control group. Furthermore, 8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin], a 5-HT(1A) receptor agonist, also induced significantly larger hyperpolarization in the ECS group than in the control group. These results suggest that repeated ECS treatment enhances function of the 5-HT(1A) receptor for 5-HT. This supports the hypothesis that enhanced 5-HT(1A) receptor function, at least in part, contributes to the effectiveness of ECS treatment for depression directly and/or indirectly.