Serum dopamine-beta-hydroxylase (DBH) activity was studied in 125 psychiatric patients and 73 normal controls. Serum DBH activity in schizophrenic, bipolar manic, and neurotic depressive patients was not significantly different from that in controls. Although serum DBH activity in unipolar and bipolar depressed patients was also not significantly different from that in controls, the subgroup of psychotically depressed unipolar patients had significantly lower serum DBH levels than controls did. Serum DBH activity in bipolar psychotically depressed patients also tended to be low, but was not significantly less than in controls. Serum DBH activity in the patient group usually did not increase in the immediate period following successful treatment with drugs or electroconvulsive therapy, indicating that basal levels of serum DBH activity in psychiatric patients are not state-dependent. Low serum DBH levels in psychotically depressed patients must be verified in an independent group of patients before its significance can be appreciated. Our findings could be the result of the study of an atypical group of psychotically depressed patients.