We have observed in the basal forebrain of the common marmoset a group of neurones which display tyrosine hydroxylase immunoreactivity (THir) with three different polyclonal antibodies and one monoclonal antibody, and which express TH mRNA as shown by in situ hybridization histochemistry. The population of cells is composed of large multipolar neurones and is located predominantly in the substantia innominata and at the ventral, medial and lateral margins of the external segment of the globus pallidus. The cell morphology and the distribution of THir cells corresponds closely to the caudal portion of the nucleus basalis of Meynert. Adjacent sections demonstrate both THir and choline acetyltransferase immunoreactivity in the cells in this group, as well as strong acetylcholinesterase activity but not dopamine immunoreactivity. These observations indicate that many cholinergic neurones in the posterior nucleus basalis of Meynert of the marmoset contain tyrosine hydroxylase, and suggest that both acetylcholine and catecholamine may be synthesised as co-localised neurotransmitters within the same magnocellular neurones. We observe no THir cells in similar areas of the basal forebrain of either rhesus or talapoin monkeys.