Nicotine has been previously shown to have immunosuppressive actions. Type I diabetes is an autoimmune disease resulting from the specific destruction of the insulin-producing pancreatic -cells. Thus, we hypothesized that nicotine may exert protective effects against type I diabetes. The multiple lowdose streptozotocin (MLDS)-induced model and spontaneous nonobese diabetic (NOD) mouse model of type I diabetes were used to assess whether nicotine could prevent this autoimmune disease. Blood glucose levels, diabetes incidence, pancreas insulin content, and cytokine levels were measured in both models of diabetes, both to asses the level of protection exerted by nicotine and to further investigate its mechanism of action. Nicotine treatment reduced the hyperglycemia and incidence of disease in both the MLDS and NOD mouse models of diabetes. Nicotine also protected against the diabetes-induced decrease in pancreatic insulin content observed in both animal models. The pancreatic levels of the Th1 cytokines interleukin (IL)-12, IL-1, tumor necrosis factor (TNF)-␣, and interferon (IFN)-␥ were increased in both MLDS-induced and spontaneous NOD diabetes, an effect prevented by nicotine treatment. Nicotine treatment increased the pancreatic levels of the Th2 cytokines IL-4 and IL-10. Nicotine treatment reduces the incidence of type I diabetes in two animal models by changing the profile of pancreatic cytokine expression from Th1 to Th2.Type I diabetes is a disease characterized by the specific destruction of the insulin-producing -cells of the pancreatic islets of Langerhans by the immune system (Bottazzo and Bonifacio, 1991;Noorchashm et al., 1997). The islet is invaded by immune cells, particularly T-cells that are CD4 ϩ and CD8 ϩ (Rabinovitch, 1994), forming a pancreatic inflammation termed insulitis, with the resulting production of immune cell mediators, such as cytokines and free radicals (including nitric oxide and related radicals such as peroxynitrite), inhibiting -cell function and ultimately inducing -cell death (Rabinovitch and Suarez-Pinzon, 1998). The induction of nitric oxide synthase (iNOS) and the subsequent formation of nitric oxide and related free radicals have been implicated in the destruction of the -cells in diabetes.A variety of cytokines have been found expressed in the insulitis lesion of the animal models of diabetes and in the pancreata of humans with type I diabetes. It has been proposed that the insulitis lesion is -cell destructive when Th1 cytokines (IL-12, IFN-␥, IL-1, and TNF-␣), produced by isletinfiltrating T-cells, dominate over Th2 cytokines (Rabinovitch, 1998). Agents that suppress the immune system, including cyclosporine (Baquerizo et al., 1989) and glucocorticoids (Like et al., 1983), have been shown to reduce the disease incidence in animal models of type I diabetes.There are two murine models of autoimmune diabetes: the multiple low-dose streptozotocin (MLDS) model and the spontaneous NOD mouse model. The MLDS model of diabetes is characterized by progressive hy...