The major challenges of efforts to reveal biological risk factors and biomarkers of depression include the complexity of underlying systems, interactions with other systems, and contextual factors governing their expression. Altered endocrine function is believed to be a central contributor to depressive illness, but across studies, evidence for a link between endocrine markers and depression has been mixed, inconclusive, or conditional in nature. In the present study, we evaluated basal

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... erone (T), cortisol (C), and CO 2 inhalation-stress-reactivity measures of these hormones (T R , C R ) as pre-deployment moderators of the later impact of war-zone stressors on depression symptoms in-theater. Materials and Methods: At pre-deployment, U.S. soldiers (N = 120) completed demographic, clinical and hormone measures, and during deployment, they completed monthly, web-based assessments of war-zone stressors and depression symptoms (N = 533 observations). Mixed effects models estimated the effects of the pre-deployment hormone profiles in moderating war-zone stressors' impact on in-theater depression. Models also tested whether hormonally linked risk for later stress-evoked depression depends on pre-existing depression. Results: Controlling for predeployment depression, high T was protective; whereas T R had depressogenic effects that were amplified by predeployment depression. Further, high C was protective, but heightened C R was depressogenic, but only among those with elevated pre-deployment depression. Conclusions: Findings highlight the importance of examining basal and reactivity measures of endocrine function, and use of prospective, longitudinal models to test hypothesized causal pathways associated with depression vulnerability in the war-zone. Results also suggest that pre-existing depression and cortisol may work in tandem to increase vulnerability for later stress-evoked depression in the war-zone.