Tyrosine hydroxylase (TOH), the rate-limiting enzyme in catecholamine biosynthesis, is regulated by phosphorylation. Activation of histaminergic H1 receptors on cultured bovine adrenal chromaffin cells stimulated a rapid increase in TOH phosphorylation (within 5 s) that was sustained for at least 5 min. The initial increase in TOH phosphorylation (up to 1 min) was essentially unchanged by the removal of extracellular Ca2+. In contrast, the H1-mediated response was abolished by preloading the cells with BAPTA acetoxymethyl ester (50 microM) and significantly reduced by prior exposure to caffeine (10 mM for 10 min) to deplete intracellular Ca2+. Tryptic-phosphopeptide analysis by HPLC revealed that the H1 response in the presence or absence of extracellular Ca2+ resulted in a major increase in the phosphorylation of Ser19 with smaller increases in that of Ser40 and Ser31. In contrast, although a brief stimulation with nicotine (30 microM for 60 s) also resulted in a major increase in Ser19 phosphorylation, this response was abolished in the absence of extracellular Ca2+. These data indicate that the mobilization of intracellular Ca2+ plays a crucial role in supporting H1-mediated TOH phosphorylation and may thus have a potentially important role in regulating catecholamine synthesis.