Introduction: The 1 alpha,25-dihydroxyvitamin D-3 [1 alpha,25(OH)(2)D-3] plays an essential role in mineral balance but has also been recognized as a powerful modulator of immune response. We aimed to examine the effect of the 1 alpha,25(OH)(2)D-3 treatment on insulin/c-peptide, catalase, superoxide dismutase (SOD), and blood glucose in rats that take streptozotocin (STZ). Methods: Forty pieces of male rats of Albino family whose average weights were 261.00 +/- 07.62 g were used in the study. Rats were made diabetic by giving STZ of 40 mg/kg during 5 days through intraperitoneal path. Some of the diabetic group and nondialbetic group were received 1 alpha,25(OH)(2)D-3. The levels of SOD, insulin, c-peptide, glucose, SOD, and catalase were measured at the zero, second, fourth, and sixth weeks. Results: Erythrocyte SOD levels didn't show a significant difference at the end of the sixth week in all groups when compared to the beginning. While erythrocyte catalase levels didn't show a significant difference in nondiabetic control and nondiabetic with vitamin D, and diabetic with vitamin D groups at the end of sixth week when compared to the beginning, a significant measurement was made in diabetic without vitamin D group. Maximal insulinitis scoring values were observed in diabetic without vitamin D that didn't receive 1 alpha,25(OH)(2)D-3 treatment. Conclusion: The highness of insulin and c-peptide levels in the group that received treatment when compared to other groups and the lowness of oxidative markers such as SOD, catalase in this study can be explained by the fact that 1 alpha,25(OH)(2)D-3 treatment prevents the intervention of apoptosis mechanism. Crown Copyright (C) 2009 Published by Elsevier Inc. All rights reserved.