Background: Atrial fibrillation (AF) is the most common arrhythmia affecting performance in horses. However, no previous studies have quantified the performance reduction in horses suffering from AF. Objectives: To quantify the effect of AF on maximum velocity (V_max), maximum heart rate (HR_max), heart rate recovery (T₁₀₀), hematologic parameters and development of abnormal QRS complexes. Animals: Nine Standardbred trotters. Methods: Two-arm controlled trial. Six horses had AF induced by means of a pacemaker and 3 served as sham-operated controls. All horses were subjected to an exercise test to fatigue before (SET1) and after (SET2) 2 months of AF or sham. The V_max and HR_max were assessed using a linear mixed normal model. Abnormal QRS complexes were counted manually on surface ECGs. Results: Atrial fibrillation resulted in a 1.56 m/sec decrease in V_max (P <.0001). In the AF group, HR_max ± SD increased from 226 ± 11 bpm at SET1 to 311 ± 27 bpm at SET 2. The AF group had higher HR_max at SET2 compared with controls (P <.0001), whereas no difference between the control and AF groups was observed at SET1 (P =.96). Several episodes of wide complex tachycardia were observed during exercise in 3 of the AF horses during SET2. Conclusions and Clinical Importance: Atrial fibrillation resulted in a significant reduction in performance, an increase in HR and development of abnormal QRS complexes during exercise, which may be a risk factor for collapse or sudden cardiac death.