Abstract: We previously reported that Chicken Breast Extract (CBEX) had an antidepressant-like effect in rats. In the present study to further clarify, the mechanism of action of CBEX, the effects of CBEX on the brain serotonin (5-HT) system was investigated in the ventral hippocampus of rats, using an in vivo micro dialysis. 5-Hydroxyindoleacetic acid, a major metabolite of serotonin, was significantly increased in animals orally administered CBEX. This result indicates that CBEX may activate 5-HT metabolism in the brain. Since, a previous study indicated that orally administered CBEX increases levels of the putative neurotransmitter carnosine in the hippocampus, carnosine might be involved in the action of CBEX.