Objective To determine the effect of exercise training on cardiac autonomic modulation in normal older adults by using analysis of heart rate variability. Subjects The exercise group consisted of 7 men and 9 women aged 66 ± 4 years. The comparison group consisted of 7 men and 9 women also aged 66 ± 4 years. Method Heart rate variability was determined from 24-hour Holter recordings before and after 12 months of supervised exercise, which consisted of 3 months of stretching and 9 months of 5 hours/week aerobic exercise at approximately 70% of maximal oxygen uptake. Heart rate variability was measured at baseline and 12 months later in the comparison group, who had not changed their usual activity level. Results In the exercise group maximal oxygen consumption increased from 1.8 ± 0.5 L/min to 2.2 ± 0.7 L/min (P < .05). The standard deviation of normal interbeat intervals increased from 126 ± 21 ms to 142 ± 25 ms. Mean nighttime heart rate decreased from 67 ± 6 beats/min to 63 ± 5 beats/min. Increased fitness level had little effect on indexes of heart rate variability, which reflect parasympathetic or mixed sympathetic/parasympathetic modulation of heart rate. There was no change in heart rate or heart rate variability in the comparison group. Conclusions Exercise training increases total heart rate variability in normal older adults. The most marked alterations are in nocturnal heart rate. Heart rate variability is stable over a 1-year period in older adults who do not alter their activity level. (Am Heart J 1999;138:567-76.)