Previously, we have identified a number of morphine- and cyclic AMP- regulated phosphoproteins (MARPPs) in the rat locus coeruleus (LC) and other brain regions. We now show that one of these phosphoproteins, a 58 kDa protein designated MARPP-58, is tyrosine hydroxylase. First, MARPP-58 comigrates with immunolabeled, immunoprecipitated, and purified tyrosine hydroxylase on 1- and 2-dimensional electrophoresis. Second, MARPP-58, immunoprecipitated tyrosine hydroxylase, and purified tyrosine hydroxylase yield identical 1-dimensional phosphopeptide maps. Third, MARPP-58 exhibits a regional and subcellular distribution in brain consistent with tyrosine hydroxylase. Identification of MARPP-58 as tyrosine hydroxylase made it possible to determine whether increases in MARPP-58 phosphorylation induced by chronic morphine in the LC reported previously are associated with alterations in enzyme activity and expression in this brain region. We show that chronic treatment of rats with morphine increases levels of tyrosine hydroxylase activity, immunoreactivity, and mRNA in the LC. Induction of the enzyme by chronic morphine was blocked by concomitant treatment of rats with the opiate receptor antagonist naltrexone, indicating that morphine produces this effect through the activation of opiate receptors. Consistent with previous observations that the chronic morphine-induced change in MARPP-58 phosphorylation is specific to the LC, changes observed in enzyme activity, immunoreactivity, and mRNA were not observed in a number of other brain regions studied. The results indicate that chronic morphine regulates the expression of tyrosine hydroxylase specifically in the LC and suggest that such regulation reflects long-term adaptations of LC neurons to chronic morphine at the level of gene expression.