The distribution of monoamine neurons in the brains of ten common marmosets (Callithrix jacchus) was examined by means of the Falck-Hillarp formaldehyde histofluorescence technique. Large populations of catecholamine and indoleamine neurons were found throughout the brain stem. Catecholamine cell bodies corresponded essentially to th noradrenaline and dopamine groups defined as A1-A7 and A8-A14, respectively. In contrast to Old World primate species, however, the noradrenaline cell populations (particularly the pontine coeruleal A6 group) were less numerous. Ascending catecholamine fibre pathways were not observed within the medulla or pons, although numerous axons were found near the mesodiencephalic border. These were fine and smooth in appearance in contrast to those of other species and this finding may represent a significant morphological difference. The catecholamine terminal innervation of the diencephalon was modest in the marmoset and was less dense than in other primates. In contrast, limbic areas and the striatum contained very large numbers of terminals. Indoleamine cell bodies, equivalent to the serotonin groups defined as B1-B9, were also observed. The most rostral cell populations (B7-9) were large. In addition, pontine and medullary indoleamine neurons extended laterally through the tegmentum as noted in other primates, such that they often adjacent to catecholamine neurons. A prominent bundle of indoleamine axons was observed in the mesencephalon and corresponded to a fibre pathway seen in rodents and other primates. No terminal varicosities were noted.