Abstract— Three days following a single injection of reserpine (10 mg/kg, i.p.) the activity and amount of dopamine-β-hydroxylase (DBH) are increased nearly 2-fold in the noradrenergic cell bodies of the nucleus locus coeruleus of rat. To determine if this increased accumulation of DBH is due to an increased rate of enzyme synthesis, [3H]amino acids were infused into the IVth ventricle of reserpine-and saline-injected rats. This method was 35 times more effective than intracisternal infusion and 600 times more effective than intravenous infusion. DBH protein was isolated from the locus coeruleus by immunoprecipitation and SDS-electrophoresis. These steps proved crucial for the complete isolation of DBH from other labelled proteins. Indeed, only 10–15% of the immunoprecipitate was finally identified as labelled DBH protein. The rate of incorporation of [3H]leucine into DBH protein of locus coeruleus was increased to 181%, of control following reserpine, whereas that into TCA-precipitable protein was unchanged. A similar result was obtained using [3H]lysine. In contrast, the apparent half-life of the enzyme did not change following reserpine. The relative rate of synthesis of DBH ([3H]DBH/3H-total protein), denoting selectivity of response, was increased in the locus coeruleus of reserpine-treated rats to 154% of control (P < 0.01). These findings indicate that increased synthesis accounts for the observed increase in DBH protein in the locus coeruleus following reserpine administration.