Tyrosine hydroxylase is considered to be the rate-limiting enzyme in the synthesis of catecholamines in both the central and peripheral nervous system. Increased or decreased neuronal activity, stress, lesions, drug effects, endocrinological manipulations and experimental models of hypertension are associated with alterations in tyrosine hydroxylase activity in the central nervous system. In many of these instances, the changes in the activity of tyrosine hydroxylase in the central nervous system that occur are localized to discrete catecholaminergic pathways and nuclei in the brain. The purpose of this review is to summarize and assess this information and to provide insight into the function of catecholamine systems in the brain and their interactions with other putative neurotransmitter systems.