The primary goal of these studies is an understanding of the regulation of tyrosine hydroxylase in the central nervous system. We have previously shown an activation of the soluble enzyme by phosphorylation or catecholamine removal. Tyrosine hydroxylase activity is now being studied in a synaptosomal system, which provides conditions more similar to those found in vivo. Synaptosomes prepared from mouse whole brain and rat striatum have been used to study the regulation of tyrosine hydroxylase. Removal of calcium and chronic morphine treatment decreases tyrosine hydroxylase activity in synaptosomes from mouse whole brain but not in those from striatum. Conversely, addition of the calcium ionophore A23187, results in increased tyrosine hydroxylase activity in synaptosomes from both brain and striatum. We have also found that tyrosine hydroxylase activity in whole brain synaptosomal preparations is dependent on extrasynaptosomal tyrosine concentration, and that removal of calcium changes the kinetics of tyrosine hydroxylase for tyrosine. tyrosine hydroxylase activity in striatal synaptosomes did not show this tyrosine dependency.