Results of electrophysiological single-cell recording studies suggest that most, if not all, types of antidepressant treatments increase 5-hydroxytryptamine (5-HT) neurotransmission. Tricyclic antidepressants, electroconvulsive shock treatment, mianserin, adinazolam, and possibly sleep deprivation may exert their therapeutic effect through sensitization of postsynaptic neurons to 5-HT. Serotonin reuptake blockers may relieve depression through an increased efficacy of the presynaptic element resulting from a desensitization of somatodendritic and terminal 5-HT autoreceptors. Similarly, monoamine oxidase inhibitors may act by increasing the efficacy of 5-HT neurons. Intensification of 5-HT function appears to be a common denominator to antidepressant treatments; however, evidence suggests that this modification may only be a link in a chain of events leading to an antidepressant response.