This study would be the first to demonstrate an effect of electroconvulsive therapy on the 5-HT1A receptor binding in humans in vivo. Given the involvement of the 5-HT1A receptor in the pathophysiology of mood disorders, the present study would be an important step towards a better understanding of antidepressant treatment and treatment response. By comparing treatment effect and the underlying biological mechanism, the study might help to identify biomarkers that distinguish patients who are likely to benefit from ECT from patients who will rather be non-responders. Finally, by investigating the role of the 5-HT1A receptor in ECT, is highly discussed relevance for antidepressant action will be further elucidated and might prepare the ground for new therapeutic strategies.