Antibodies to the catecholamine synthetic enzymes tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH), and phenylethanolamine-N-methyltransferase (PNMT) were used in an immunohistochemical analysis of the brain of the golden hamster. The distributions and morphological characteristics of neurons displaying immunoreactivity to these enzymes were examined in sets of adjacent sections. Various novel groups of TH-immunoreactive neurons were found. A distinct feature observed in the hamster brain was the presence of a population of magnocellular multipolar neurons in the basal forebrain which displayed intense TH immunoreactivity. These cells were found predominantly in the vertical and horizontal limbs of the nucleus of the diagonal band of Broca and in the lateral preoptic area. Many small TH-positive cells were also found scattered in the deeper layers of the cortex in the hamster. The pericentral divisions of the inferior colliculus contained a large number of TH-immunoreactive neurons, and a few small bipolar cells in the lateral superior olive were also stained. A major cell group was found in the lateral parabrachial nucleus at the level of the locus ceruleus that displayed TH but not DBH immunoreactivity and was obviously separate from the TH- and DBH-positive cells of the locus ceruleus. Additional TH-positive cell groups were found along the seventh nerve, within the medial longitudinal fasiculus, in the nucleus raphe pallidus, and in the pars caudalis of the spinal trigeminal nucleus. The various catecholamine cell groups described by many people in the rat by use of histochemical and immunohistochemical techniques were also present in the hamster brain. These included the noradrenergic, TH- and DBH-immunoreactive cell groups of the pons and medulla. The hamster also displayed groups of medullary neurons displaying immunoreactivity to TH, DBH, and PNMT. These appeared similar in distribution and morphology to the adrenaline cell groups described in the rat. TH-immunoreactive cell groups in the olfactory bulb, hypothalamus, substantia nigra, and ventral tegmental area of the hamster appeared to correspond to the dopaminergic cells groups described in the rat and other species. In addition, as in the rat and cat, numerous TH-positive cells were found in the dorsal motor nucleus of the vagus, the nucleus of the solitary tract, and the area postrema. These observations suggest that catechols may be present in neurons in the cortex, basal forebrain, auditory brainstem, and the parabrachial nucleus of the hamster. These studies also emphasize the need for caution in making generalizations regarding transmitter distributions across species.