The distribution and morphology of presumed dopaminergic neurons within the reticular formation (RF) and the ventrolateral tegmental area (VLT) were studied by using a specific antibody to the enzyme that converts tyrosine to dihydroxyphenylalanine, tyrosine-hydroxylase (TH), in combination with a sensitive immunoperoxidase method (Hsu et al., '81). Incubation of thick (70-120 micron) sections for 3-5 days in high dilutions of antibody resulted in staining of TH-immunoreactive neurons in a Golgi-like fashion. Analysis of serial sections cut in the coronal, horizontal, and parasagittal planes revealed an extensive system of TH-positive neurons in the RF and VLT extending from the Edinger-Westphal nucleus caudally to the level of the decussations of the superior cerebellar peduncle. Within this region, the TH-positive cells belong to two subgroups: (1) a relatively well-defined population of cells aggregated in the reticular formation (corresponding to cell group A8 of Dahlström and Fuxe, '64), and (2) a more loosely defined group of cells that appears to be continuous with the cells of the nucleus raphe linearis. This latter cell group extends laterally from the midline to the nucleus parapeduncularis. An analysis of the individual TH-immunoreactive cells revealed large differences in their morphology. Thus, the somata of TH-positive cells in the RF and the VLT are fusiform, ovoid, or triangular. A majority of the TH neurons are of medium (long axis: 15-35 micron) to large (long axis: 35-40 micron size. While the cells in the A8 area appeared relatively homogeneous, the TH-positive cells of the VLT showed great variations in dendritic branching pattern and orientation. Taken together, the present study has shown that within the RF and the VLT, the TH-immunoreactive neurons are more numerous than hitherto recognized, and that this cell group consists of a morphologically heterogeneous population of dopamine-synthesizing neurons.