Serum dopamine-β-hydroxylase (DBH) activity was tested in 160 psychiatric patients suffering from affective disorders. The patients were divided into subgroups according to strict diagnostic criteria and clinical state at the time of the examination. Since no relationship was found between DBH activity and age or sex, these variables have been discarded from further analyses. Bipolar (manic-depressive) patients had lower (p < 0.05) DBH mean values than patients suffering from unipolar, recurrent depressive psychoses. No other differences were found either among groups or in relation to the clinical state. As such a wide range of DBH values was recorded in all the groups examined, measurements of DBH activity do not seem to be useful as a diagnostic aid, or as an indicator of the clinical state of patients suffering from affective disorders