Chronic electroconvulsive shock (ECS) is known to increase the level of serotonin-2 (S2) receptors in male rat brain. Using quantitative autoradiography, we have studied the distribution pattern of these receptors in female as well as male rats and the effect of repeated ECS on the receptor level in both sexes. We find that although the distribution of S2 receptors is generally similar in males and females, they respond differently to repeated ECS. In males we found the expected increase in S2 binding, which was localized to specific cortical, hippocampal, and septal regions. In females, no increase was found in the cortex or septum and relatively small increases were found in the hippocampus. It appears that the regulation of S2 receptors by ECS is sex-dependent.