These studies examined the effects of reserpine on concentrations of norepinephrine (NE), dopamine (DA) and epinephrine (EPI) and on levels of tyrosine hydroxylase (TH) mRNA in locus coeruleus (LC) and medullary A1 and A2 neurons. Noradrenergic neurons in these regions first were identified by immunocytochemistry and, thereafter, by in situ hybridization histochemistry. Levels of TH mRNA were measured by quantitative image analysis methods. Changes in catecholamine concentrations in micropunches of these brain regions were analyzed by HPLC. Epinephrine was not detected in any of the nuclei examined. Twenty-four hours after reserpine treatment, NE concentrations declined in A1, A2 and LC neurons by 46, 69 and 34% respectively while DA declined only in the region of A2 neurons. This reserpine-induced depletion of NE was accompanied by a 2- to 3-fold increase in TH mRNA levels in LC and A1 neurons but no change in message levels occurred in A2 cells 24 h after reserpine. Forty eight hours later, message levels in A1 and LC neurons did not differ significantly from the elevated 24 h values but TH mRNA levels in A2 neurons now were significantly elevated compared to 24 h values. TH mRNA levels 72 h after reserpine did not differ from 48 h values in A1, A2 and LC neurons. Thus, TH gene expression in A1 neurons increases after reserpine treatment in a manner equivalent to that observed in LC, adrenal medulla and superior cervical ganglia. The reason why it required 48 h for TH mRNA to increase in A2 neurons remains unclear.