We investigated the effect of electroconvulsive treatment (ECT) on platelet 14C-serotonin uptake, 3H-paroxetine binding and 5-HT2 receptors in 12 patients (10 women and 2 men) unresponsive to pharmacological treatment. The mean numbers of ECTs given was 6.1 ± 1.5. Mean treatment days was 14.6 ± 3.8. Mean percent reduction in MADRS scores was 80.7 ± 19.7 (p <0.002). The number of 5-HT2 receptors increased significantly and uniformly after ECT (p = 0.011). There was no correlation between the degree of increase in 5-HT2 receptor densities and the reduction in MADRS scores after ECT. There was no difference in mean Bmax for platelet 3H-paroxetine binding before and after ECT. Bmax increased in six patients and decreased in six patients. The study shows an increase in platelet 5-HT2-receptor densities in depression after repeated ECT. Recognizing the similarities between 5-HT2 receptors in platelets and cerebral cortex, it seems reasonable to assume that a similar upregulation of cortical 5-HT2 receptors occurs after ECT.