Evidence indicates that chronic social stress plays a significant role in the development of cancer and depression. Although their association is recognized, the precise physiological mechanism remains unknown. In our previous work, we observed that OF1 males subjected to chronic social defiance exhibited anhedonia, and those who developed tumors in the lung showed anxiety-associated behaviors. In this study, we observed that tumor-bearing OF1 mice presented higher levels of 3-HK, and this increase may be due to IDO. No differences in hippocampal catecholamine levels were observed. Our results suggest that a systemic tumor can induce molecular changes in the hippocampal kynurenine pathway that may impact behavior.