Evidence for the genetic control of human MAO is now well established. The relationships of MAO activities to neuropsychiatric disorders or response to psychopharmacologic treatments are relatively unstudied. Preliminary findings from our own studies suggest that (1) blood MAO activity is a polygenically controlled trait; (2) there may be a subgroup of depressed patients with high MAO activity who are more severe symptomatically and more resistant to treatment, and (3) that men with atypical and mixed depressions respond more favorably than women to treatment with an MAO inhibitory drug.