Evidence from several studies supports the involvement of several neurotransmitter systems in the mechanism of action and the clinical efficacy of the electroconvulsive therapy (ECT). However, more recent studies have reported serotonin, through the activation of several receptors, to be the neurotransmitter most frequently altered in ECT. With regard to the serotonergic system, several levels of alteration can be targeted, that concerning serotonin and its metabolite concentrations, that concerning changes in the density of presynaptic and postsynaptic receptors located both in brain tissue or in platelets, and finally, alterations at the intracellular signalling system or second messenger level.