We evaluated serotonin (5HT) uptake parameters in platelets of a group of bipolar I patients in both a depressive phase and a mixed state, before (t0) and after (t1) the first electroconvulsive treatment (ECT). The results showed the presence of significant changes in the number of the carrier proteins for 5HT at t1, which in most cases preceded the clinical improvement. Although other mechanisms are certainly involved, these data suggest that ECT may influence the 5HT transport through platelet membranes.