Introduction: The purpose of this study was to investigate the ultrastructural change of the thymus under stress conditions created by diabetes accompanied by fasting, and also the effects of insulin therapy. Methods: Forty-eight Sprague-Dawley type rats were used in this experiment. Type 1 diabetes symptoms were induced in 24 of the rats by the application of a single dose of intravenous streptozotocin in sodium citrate buffer through the tail vein. A single dose of sodium citrate buffer was given to rats to create a control group. Following the infusions, rats were divided into control, control and fasting, diabetes, diabetes and fasting, and insulin treatment groups. At the end of the experiment tissues from the thymus of the rats were extracted and examined using electron microscopy. Results: Severe degeneration was observed in the prolonged fasting (stress) and diabetes groups without insulin treatment. Insulin treatment was found to mostly reverse this degeneration. Conclusion: This study demonstrates that the thymus was affected ultrastructurally by diabetes and concomitant fasting, and insulin treatment can reverse these changes. © Springer Healthcare Communications 2008.