The aim of our study was to compare the tolerance to isometric and isotonic exercises by determining if an improvement would exist, or not, in the cardiovascular response to these exercises after reverting to sinus rhythm in patients with atrial fibrillation. Twenty seven cases with nonvalvular chronic (having periods ranging between 48 hrs and 1 year) atrial fibrillation, 14 female and 13 male (mean age 63.10 ± 11.85 years) were included in the study. Sinus rhythm could not be restored in two cases. The investigation was carried on the remaining 25 cases. Isometric and isotonic exercise tests were performed with all of the patients before and 48 hours after the rhythm was reverted to sinus. Isometric exercise test was performed by handgrip test, and isotonic exercise test was performed immediately after the isometric exercise test by using modified Bruce protocol, limited by symptoms. The electrocardiograms were recorded, and heart rates and blood pressures were measured at rest, at each stage of the exercise, at maximum effort and at every minute of the recovery period. Repeated measures of ANOVA test and linear regression test were used for statistical evaluations. Heart rate at rest was significantly low after reverting rhythm to sinus at isometric exercise (p=0.0001). This decrease continued to be significant at the 1st, 2nd and 3rd minutes of the test (p=0.0014, p=0.0002 and p<0.0001, respectively). Although there were no significant difference between pressure-heart rate products at rest, this value was significantly higher at the 1st, 2nd and 3rd minutes before reverting rhythm to sinus (p=0.049, p=0.048 and p=0.022, respectively). Also, no significant difference was determined between systolic and diastolic arterial pressures neither at rest, nor during exercise. In isotonic exercise, heart rate at rest (p=0.0015), at 1st, 2nd and 3rd minutes of the 1st stage (p<0.0001, for all), at the end of exercise (p<0.0001); the increase in heart rate at 1st, 2nd and 3rd minutes of the 1st stage and at the end of the exercise (p<0.0001, for all); pressure-heart rate products at rest, at 1st, 2nd and 3rd minutes of the 1st stage and at the end of the exercise (p<0.0001, for all) were all found to be significantly low after restoring sinus rhythm. The exercise periods and MET values of the patients increased significantly after sinus rhythm was restored (p=0.0014 and p=0.0054, respectively). Hence, restoring sinus rhythm by cardioversion in patients with atrial fibrillation improves excessive heart rate response to exercise, for both isometric and isotonic exercises. Effort capacities of the cases also increased significantly.