Incubation of dopamine-4-O-sulfate with purified bovine dopamine-.beta.-hydroxylase led to the formation of free norepinephrine [NE]. The production of free NE was completely inhibited in the presence of 5.6 mM of fusaric acid, an inhibitor of dopamine-.beta.-hydroxylase. No sulfatase activity was detected in the incubation medium. The reaction of dopamine-4-O-sulfate with dopamine-.beta.-hydroxylase had an apparent Km of 2.6 mM.