In the present study we report the results of investigations into the serum zinc levels in a clinical study of 19 patients with unipolar depression; 16 normal controls and in three animal models of depression: chronic severe stress (CSS), chronic mild stress (CMS) and olfactory bulbectomy (OB) in rats. CSS model and unipolar depressed patients exhibit lower serum zinc levels than the appropriate controls. There was no effect on the value in CMS or OB models. The present findings confirm previous clinical data indicating decreased serum zinc levels in human depression. Moreover, the data indicate that differences exists in the serum zinc levels in different animal models of depression and suggest a similarity between the CSS model and human depression with regard to this marker. Copyright © 1999 John Wiley & Sons, Ltd.