Immunocytochemical demonstration of monoamine oxidase-A, monoamine oxidase-B and tyrosine hydroxylase was performed in the human brain stem using monoclonal antibodies to monoamine oxidase-A and monoamine oxidase-B and polyclonal antibodies to tyrosine hydroxylase. In most of the brain areas examined, except the serotonergic dorsal nucleus of raphe, the noradrenergic locus coeruleus and the dorsal efferent nucleus of vagus, tyrosine hydroxylase-positive neurons were in greater number than monoamine oxidase-A-stained or monoamine oxidase-B-stained neurons. The dorsal nucleus of raphe showed no tyrosine hydroxylase immunoreactivity, but reacted positively to serotonin- and monoamine oxidase-B antibodies, while monoamine oxidase-A staining was moderate. In none of the investigated brain areas did neurons exclusively react with monoamine oxidase-B antibodies without expressing monoamine oxidase-A in a few neurons, while in some areas neurons expressed both monoamine oxidase-A and tyrosine hydroxylase (locus coeruleus; dorsal efferent nucleus of vagus). The oculomotor nucleus stained only with monoamine oxidase-A antibodies, substantia nigra neurons reacted only with tyrosine hydroxylase antibodies. Glial staining in most of the brain areas examined seemed, with slight differences, to have the same intensity with monoamine oxidase-A and monoamine oxidase-B antibodies used. No glial staining was obtained with tyrosine hydroxylase antibodies.