The primary goal of these studies is an understanding of the regulation of tyrosine hydroxylase in the central nervous system. Previously we have shown that phosphorylation or catecholamine removal activates the soluble enzyme. However, other factors may regulate tyrosine hydroxylase through mechanisms requiring the in vivo environment. Therefore, we are studying the rate of tyrosine hydroxylation in a synaptosomal system. Synaptosomes prepared from mouse whole brain, rat striatum and rat cortex have been used to study the regulation of tyrosine hydroxylase. Addition of calcium to the media increases tyrosine hydroxylase activity in synaptosomes from mouse whole brain and rat cortex, but not rat striatum. In addition, we have found that addition of calcium changes the kinetics of tyrosine hydroxylase for tyrosine in whole brain and cortical synaptosomes. Tyrosine hydroxylase activity in striatal synaptosomes did not show this tyrosine dependency.