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.