In treatment-resistant depression, electroconvulsive therapy (ECT) has been shown to effectively reduce depressive symptoms, though the underlying neurobiological mechanism is still unclear. The serotonergic system, and in particular the inhibitory serotonin-1A (5-HT1A) receptor, appears to be significantly involved in the effectiveness of ECT. The aim of the study is to assess the effects of ECT on the 5-HT1A receptor binding potential (BPND) and distribution in humans in vivo using positron emission tomography (PET) and the radioligand [carbonyl-11C]WAY-100635.