To know the effects of nicotinamide (NCT) treatment for 5 weeks at the early age on insulitis and development of diabetes in non-obese diabetic (NOD) mice, this experiment was performed. Ten ICR (Institute of Cancer Research) and 15 female NOD mice at 4 weeks of age were used. Mice were assigned to ICR and NOD groups, and NOD mice were randomly divided to control and NCT-treated groups. NCT was administered to mice orally as a solution and in a dose of 500 mg/kg body weight a day from the age of 4 to 8 weeks. Diabetes onset was 18 weeks of age in control group, and 22 weeks of age in NCT-treated group. Cumulative incidences of diabetes at 25 weeks of age in control and NCT-treated NOD mice were 63 and 29%, respectively. Insulitis occurred in all NOD mice. Incidence of insulitis in total islets was decreased by NCT treatment in diabetic NOD mice, but intensity of insulitis was not improved by NCT treatment. Blood glucose level was increased markedly, and plasma insulin level was decreased by diabetes development in NOD mice. Plasma triglycerides and total cholesterol levels were increased in diabetic mice than in non-diabetic mice. In conclusion, these results suggest that NCT treatment for 5 weeks at the early age in NOD mice inhibits development of diabetes and insulitis in diabetic NOD mice.