The expression of tyrosine hydroxylase mRNA in the rat brain was examined using a novel alkaline phosphatase labelled antisense oligodeoxynucleotide probe. The alkaline phosphatase labelled probe revealed the presence of tyrosine hydroxylase mRNA in all the major cell groups and cell bodies previously described as containing catecholamine fluorescence or known to contain tyrosine hydroxylase immunoreactivity. Using standardized development protocols qualitative comparisons between the amount of mRNA signal in different adrenergic, noradrenergic or dopaminergic cell groups could be made. These studies showed that of the three known catecholaminergic cell types the level of tyrosine hydroxylase mRNA signal was high in the noradrenergic and dopaminergic cells, but much lower in the adrenergic cell groups. The sensitivity of this nonradioactive method of in situ hybridization is excellent and has considerable potential for studies of coexistence or coexpression of two mRNA signals for the localization of mRNA signals at the electron—microscope level.