Among a variety of factors known to affect the activity of tyrosine hydroxylase, catecholamines, cyclic AMP-dependent protein kinase, calmodulin-dependent protein kinase II, and polyanions resulted in the reversible modulation of the enzyme activity to a greater extent than the other factors. The in vitro experiments suggested that the inactivation of the enzyme by catecholamines, the end products of the enzyme, and the activation by cyclic AMP-dependent protein kinase might be most important in controlling the activity of the enzyme. Incubation of the enzyme with catecholamines at a concentration as low as 10-100 nM resulted in a rapid inactivation, and the inactivated enzyme was found to be more stable than the original uninactivated enzyme. The inactive/stable form of the enzyme exhibited no activity under the physiological conditions. Several lines of evidence indicated that tyrosine hydroxylase might usually exist as the catecholamine-induced inactive/stable form in the nervous system. This inactive form of the enzyme was markedly activated by cyclic AMP-dependent protein kinase. This conversion of the enzyme induced by cyclic AMP-dependent protein kinase was a dramatic one from the almost completely inactive form to the extremely active form having an activity as high as 2,800 units per mg of protein, which was the highest so far reported when the enzyme activity was measured at the physiological pH (pH 7). The co-operative action of cyclic AMP-dependent protein kinase, calmodulin-dependent protein kinase II, and polyanion were also examined. Most of the studies on the regulatory mechanism for tyrosine hydroxylase so far reported have been done with the enzyme from the rat. Tyrosine hydroxylase from human pheochromocytoma was found to be remarkably activated by cyclic AMP-dependent protein kinase. Calmodulin-dependent protein kinase II and polyanion also modulated the activity of the human enzyme. These results suggest that tyrosine hydroxylase may be regulated in a similar manner in the human and the rat.