Background: Critical load (CL) indicates the transition from moderate dynamic exercise to intense exercise and can be useful as a parameter of exercise intolerance during resistance exercise. In this sense, this study aimed to assess and contrast the cardiac autonomic response by heart rate variability during resistance exercise at CL in elderly and young individuals. Methods: Twenty apparently healthy active men, were allocated into young group (n=10) and elderly group (n=10), who underwent: 1) one repetition maximum (1RM) test on the Leg Press at 45º; 2) constant load exercise tests (70%, 80% and 90% of 1RM and intermediate load) in order to calculate the CL; and 3) CL assessment with record of heart rate (HR) and R-R intervals (R-Ri) to heart rate variability assessment. Results: During CL, the elderly presented lower value of mean HR in relation to young group (p<0.05), and mean R-Ri, RMSSD and SD1 were higher in elderly compared to young (p<0.05). In addition, delta (exercise minus rest) mean HR, STDRR, RMSSD, RRtri, TINN, SD1, SD2 and correlation dimension were lower in elderly compared to young (p<0.05). Conclusion: Elderly presented lower response to vagal withdrawal and less complexity compared to the young people during resistance exercise at CL.