INTRODUCTION: Decreased heart rate variability (HRV) in patients with end stage renal disease (ESRD) undergoing hemodialysis is predictive of cardiac death, especially due to sudden death. OBJECTIVE: To evaluate the effects of aerobic training during hemodialysis on HRV and left ventricular function in ESRD patients. METHODS: Twenty two patients were randomized into two groups: exercise (n = 11; 49.6 +/- 10.6 years; 4 men) and control (n = 11; 43.5 +/- 12.8; 4 men). Patients assigned to the exercise group were submitted to aerobic training, performed during the first two hours of hemodialysis, three times weekly, for 12 weeks. HRV and left ventricular function were assessed by 24 hours Holter monitoring and echocardiography, respectively. RESULTS: After 12 weeks of protocol, no significant differences were observed in time and frequency domains measures of HRV in both groups. The ejection fraction improved non-significantly in exercise group (67.5 +/- 12.6% vs. 70.4 +/- 12%) and decreased non-significantly in control group (73.6 +/- 8.4% vs. 71.4 +/- 7.6%). CONCLUSION: A 12-week aerobic training program performed during hemodialysis did not modify HRV and did not significantly improve the left ventricular function.