The cellular localization of neurons expressing cholecystokinin (CCK) and tyrosine hydroxylase (TH) mRNAs was analysed in rat ventral mesencephalon using in situ hybridization techniques with both complementary DNA and synthetic oligonucleotide probes. Cell bodies distributed throughout the substantia nigra, ventral tegmental area, interfascicular nucleus, midline raphe nuclei, and central and ventral periaqueductal grey matter were found to contain CCK mRNA or TH mRNA as indicated by high densities of grains overlying the perikarya. The in situ hybridization technique was combined with immunocytochemistry on the same tissue section to localize the peptide or enzyme within its respective mRNA-containing somata. In addition, the presence of TH immunoreactivity was demonstrated within cell bodies labeled for CCK mRNA and immunostaining for CCK was detected within TH mRNA-containing neurons. In the medial geniculate nucleus a strong labeling for CCKmRNA was observed, in spite of the fact that so far no CCK-like immunoreactivity has been demonstrated in perikarya in this nucleus. The specificity of the probes was verified by RNA blot hybridization. These results confirm recent double-labeling immunocytochemical studies and further characterize the coexistence of CCK and TH at the level of their mRNAs as well as their post-translational products in a large population of mesencephalic dopamine neurons known to project to forebrain areas.