Tyrosine hydroxylase catalyzes the rate-limiting step in the catecholamine biosynthetic pathway. Short-term TH activity is proposed to be regulated by the phosphorylation/dephosphorylation of regulatory domains Ser 40, 31, and/or 19 in response to membrane depolarization coupled increase in intracellular Ca2+. Here, we present in situ evidence to support that extracellular H+ ions ([H+]o) are an intracellular or extracellular Ca2+-independent novel signal for TH activation in catecholaminergic MN9D and PC12 cells. [H+]o-mediated TH activation is a short-term process coupled with a Na+ - independent Cl?/HCO3 ? exchanger-mediated increase of intracellular hydrogen ions ([H+]i). While extracellular Ca2+ is not required for [H+]o-mediated TH activation, [H+]o does not increase the cytosolic Ca2+ levels in neuronal or non-neuronal cells in the presence or absence of extracellular Ca2+. Although [H+]o-mediated TH activation is associated with a significant increase in Ser 40 phosphorylation, major protein kinases proposed to be responsible for this process appear to be not involved. However, we have not been able to identify the protein kinase(s) involved in [H+]o-mediated phosphorylation of TH at present. Studies with a panphosphatase inhibitor, okadaic acid (OA), appear to indicate that the inhibition of phosphatase activities may not play a significant role in H+ -mediated activation of TH. The relevance of these findings to the physiological TH activation mechanism and hypoxia, ischemia, and trauma-induced selective dopaminergic neural death is being discussed in this paper.