The coexistence of calretinin (CR), a calcium binding protein. and tyrosine hydroxylase (TH)1 the rate.limiting enzyme in the synthesis of dopamine, was studied in the rat substantia nigra (SN) and ventral tegmental area (VTA). Using double-labelling immunofluorescence, it was possible to distinguish three distinct cell types: cells immunoreactive for CR only (CR+)1 cells immunoreactive for TH only (TH+) and cells in which both proteins were localized (CR+TH). The number of CR+ cells in the SN compacta region was very low, constituting less than 10% of the total cells counted throughout the rostro-caudal extent of the region. TH+ cells and colocalized cells (CR+TH) compromised approximately 45% each of the total number of cells counted. In contrast, the number of TH+ cells in the SN reticulate was very low in rostral sections (less than 10%) but rose in number in more caudal sections (to approximately 20%). CR+ cells averaged approximately 60-70% in the reticulate region. Colocalized cells in this area were infrequent (less than 20%). In the VTA, the number of TH+ cells was nearly 50% of total cells counted, compared with 15-20% for CR+ and 30-35% for colocalized cells. When examining only the cells which contained CR, the percent of these cells which also contained TH was approximately 80% in the SN compacta and 65% in the VTA. In contrast, the percent of TH cells which contained CR was closer to 50% in the SN compacta and 40% in the VTA. Colocalization levels did not appear to differ between these two types of cells in the SN reticulate (close to 50%). Taken in the context of calcium-induced neurodegeneration of dopamine- containing cells, the presence of CR within cells containing TH may be considered as possibly neuroprotective. As such, changes in CR should be contemplated in Parkinson's disease.