Km and V of serum dopamine-beta-hydroxylase and platelet monoamine-oxidase (MAO) were determined in the depressive and manic state as well as after recovery in patients with affective psychoses and in the depressive state of patients with neurotic depression. DBH and MAO values were measured also in age- and sex-matched controls. The Km and V values of each enzyme failed to differ between patients and healthy controls or between patients during and after an episode of illness. The ratio of V of serum DBH (tyramine as substrate) tended to be lower in patients independent of their psychopathological state than in controls. The findings fail to support the notion that characteristics of the peripheral enzymes measured provides a reliable indicator of a biological disposition to depression, but are consistent with the notion that the balance of monoamine may play a role in certain psychopathological states.