The non-obese mouse model of autoimmune diabetes (NOD mouse), exhibits a strain-dependent preponderance of disease in females. Castration of male NOD mice leads to an increased incidence of diabetes, suggesting that testosterone directly modulates the expression of diabetes in the NOD mouse. However, castration also modulates hypothalamic and pituitary hormone production via removal of the negative feedback effects of testosterone. One hypothalamic hormone with immunomodulatory properties whose expression is increased by castration is gonadotropin-releasing hormone (GnRH). To test whether the increased incidence of diabetes in castrated male NOD mice is related to an increase in GnRH activity, we treated castrated male NOD mice with Antide, a GnRH receptor antagonist, to determine the effect on the incidence and timing of onset of diabetes. The prevalence of diabetes at 40 weeks of age in male NOD mice was 50% in sham operated mice, compared with an 83% prevalence in castrated males. Antide administration prevented the increased incidence of diabetes in the castrated male mice. Antide reduced total serum IgG levels, IL-6 cytokine expression in cultured splenocytes, and the lymphocytic infiltration of islets. GnRH administration exerted reciprocal effects, leading to earlier timing of onset of diabetes and increases in serum total IgG levels. We conclude that GnRH modulates the expression of diabetes in the NOD mouse independently of gonadal steroids.