The authors describe the anatomical features and physiological properties of central serotonergic neurons. The central serotonin neurons (part of which store peptides [substance P, TRF, enkephalins] in addition to 5HT) are highly collateralized reticular-type brain stem neurons receiving multi-modal afferent information from ascending sensory and descending motor pathways. They are under control by noradrenergic, peptidergic and and gaba-ergic projection neurons and interneurons. Furthermore, they establish variable synaptoid and synaptic contacts to neuronal, glial and secretory targets throughout the entire neuraxis and send terminal branches into the ventricular CSF space. Firing rate and transmission activity appear to be controlled in a complex and rather rigid manner by 5HT release-dependent dendrodendritic and dendrosomatic inhibition via autoreceptors (which also regulate release at the axon terminals) and via transsynaptic inhibitory feedback circuits which may involve gabaergic projection and interneurons. 3H-imipramine appears to bind to an "imipramine recognition site" in the vicinity of the 5HT carrier, and to a variety of other transport and (postsynaptic) receptor sites (NA uptake, H1, 5HT2- and alpha 1-binding sites). Circumstantial evidence points to an as yet undetermined role of the postsynaptic 5HT-1-binding sites in neurotransmission. 5HT-2-binding sites fulfil the criteria for receptors: binding affinity of antagonists to these sites correlates significantly with their potency to inhibit behavioral excitation in rats elicited by 5-hydroxytryptophan or 5HT agonists.(ABSTRACT TRUNCATED AT 250 WORDS)