Background: The increasing number of horses diagnosed with insulin resistance (IR) and the suggested link between IR and laminitis has highlighted the need to accurately diagnose IR in clinical practice.Hypothesis/Objectives: The aim of the study was to evaluate the repeatability of the combined glucose-insulin tolerance test (CGIT) as well as to determine the effect of 2 different breeds and the effect of a stressor on the test results.Animals: Clinically normal horses, 9 Standardbred horses and 9 Icelandic horses. Methods: Prospective clinical nonrandomized trial. The CGIT was performed on all horses on 2 occasions 3 weeks apart. An additional CGIT was performed on four of the Standardbred and four of the Icelandic horses after transportation to a new environment (stressor) the day before testing.Results: Calculated parameters for the glucose curve of the CGIT had low repeatability, whereas the parameters for the insulin curve had high repeatability. There was an effect of breed (Standarbred versus Icelandic horse) as well as stress on the glucose dynamics, but not on the insulin dynamics of the CGIT.Conclusion and Clinical Importance: Repeatability of the glucose dynamics of the CGIT is low. In addition, there appears to be breed differences in the glucose dynamics. It is therefore suggested that diagnosis of IR should not be made solely based on results from the glucose curve of the CGIT.