A review of the data concerning the role of 5-hydroxytriptamine (5HT)-ergic and noradrenergic (NA) system in the regulation of behavior. Reciprocal effects of cerebral 5HT-ergic and NA-ergic systems on brain morphogenesis and on behavior suggest that normal brain ontogenesis is mediated by the steady state of 5HT and NA. The disbalance between 5HT-ergic and NA-ergic systems in brain structures results in abnormal-brain development and behavioral pathologies. The concept of reciprocal effects of 5HT-ergic and NA-ergic brain systems may serve as a basis for new principles of pharmacological correction of pathologies resulting from inborn or acquired disbalance of monoaminergic systems.