Type 1 diabetes is an autoimmune disease that results from the destruction of the insulin-producing pancreatic beta islet cells, probably via the influence of cytokines. However, direct correlation between the expression of selected cytokines by various immune cells at different time points during the progression of the disease has not yet been clearly demonstrated. In this study, we showed that the mRNA expression of the pro-inflammatory cytokines, TNF-alpha, IL-1 beta, IL-6, and GM-CSF, were increased while the anti-inflammatory cytokine, TGF-beta, decreased in the peritoneal macrophages of nonobese diabetic (NOD) mice. IL-6 expression however decreased when the mice became diabetic. Surprisingly the expression of IFN-gamma and IL-2 by splenic CD4+ cells were lower in 5-week-old NOD mice as compared to the nonobese diabetic resistant (NOR) control mice, but their expression was higher in older NOD mice. The expression of IL-4 and IL-10 decreased in splenic CD4-positive lymphocytes. Splenic CD8-positive lymphocytes expressed increased levels of IFN-gamma and IL-10 but the latter decreased sharply when diabetes occurred. The relevance of these findings to the pathogenesis of type 1 diabetes is discussed.