In gonadectomized (GDX) animals replaced with subcutaneous steroid implants supplying physiological levels of testosterone (T; 1–10 ng/ml), the magnitude of adrenocorticotropic hormone (ACTH) and corticosterone (B) responses to restraint was negatively correlated with the level of T replacement, reflecting the inhibitory influence of T on hypothalamic-pituitary-adrenal (HPA) responses to stress. Although T had no effect on resting-state levels of corticotropin-releasing hormone (CRH) in the median eminence, arginine vasopressin (AVP) levels were significantly lower in GDX animals replaced with higher T levels, and the magnitude of the ACTH response to restraint was strongly correlated with median eminence levels of AVP. High physiological levels of T increased glucocorticoid receptor binding in the medial preoptic area (MPOA), with no effect on mineralocorticoid receptor binding or on glucocorticoid receptor binding in other regions. Crystalline T or B implants in the MPOA significantly reduced plasma ACTH and B responses to 10 min of restraint stress compared with cholesterol-implanted controls. Moreover, B or T MPOA implants also decreased resting-state levels of AVP but not CRH in the median eminence, and these effects were correlated with ACTH responses to restraint. Finally, lesioning the MPOA blocked the inhibitory effects of high peripheral T levels on ACTH and B responses to restraint. Thus, variations in the magnitude of HPA responses to stress among males are explained in part by individual differences in circulating T levels, and the MPOA is a critical site for this effect via the inhibition of hypophysial AVP.