SK3 small conductance, calcium-activated potassium channels play an important role in regulating the activity of mesencephalic dopamine (DA) neurons. In the present series of experiments, in situ hybridization techniques were used to compare SK3 and tyrosine hydroxylase (TH) mRNA expression throughout the rostrocaudal extent of the ventral midbrain in juvenile and adult rats. SK3 mRNA was found exclusively in areas that also contained large numbers of DA neurons including the substantia nigra (SN), the ventral tegmental area, and related cell groups (VTA-A10). An anteroposterior and mediolateral gradient in SK3 mRNA hybridization was apparent in the VTA-A10 but not in the SN. Younger rats appeared to possess higher levels and less regional variation in TH and SK3 transcripts. These results are consistent with previous studies reporting differential expression of SK3 protein within the midbrain and suggest that variations in SK3 channel distribution could contribute to differences in dopamine-related functions in the rat.