The effects of a single or repeated treatment with electroconvulsive shock (ECS) or imipramine on the central serotonin (5-HT) uptake binding sites were studied in the rat frontal cortex and hippocampus. The selective 5-HT uptake inhibitor citalopram and clomipramine potently inhibited the binding for [3H]paroxetine (5-HT uptake binding sites) in the frontal cortex. The antidepressant drugs imipramine and desipramine inhibited the binding moderately, but the 5-HT-related agents, 5-HT, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), mianserin and ketanserin inhibited it weakly. A single ECS increased the density of [3H]paroxetine binding sites, but did not alter the affinity, after 1 or 24 h, in the frontal cortex. Repeated treatment with ECS, but not with imipramine, increased the density of [3H]paroxetine binding sites in the same region. The hippocampal [3H]paroxetine binding did not change after any of these treatments. These results suggest that a single treatment with ECS causes a rapid increase in the neuronal 5-HT transporter complex and the increase lasts for 14 days in the frontal cortex.