The effect of smoking cessation on cardiac autonomic tone, as reflected by indexes of heart rate variability (HRV), has not been reported. Current smokers (n = 54, mean +/- SD age 43 +/- 12 years) who desired to quit, and were smoking > or = 1 pack/day and had made > or = 1 prior attempt at quitting, had 24-hour electrocardiographic recordings. They then attended smoking cessation classes and used transdermal nicotine patches while abstaining from smoking. After 4 to 6 weeks of using 21 mg patches, the 24-hour electrocardiogram was repeated (n = 35). Four weeks after cessation of patch use, the 24-hour electrocardiogram was again recorded in subjects who continued to be abstinent (n = 25). Time and frequency domain measures of HRV based on normal R to R (NN) intervals were computed for all recordings. Smoking cessation significantly decreased heart rate, and increased all 24-hour time and frequency domain indexes of HRV. Part of this change occurred in the transition from smoking to the patch, and further changes occurred with cessation of patch use. For example, the standard deviation of average NN intervals was 114 +/- 28 ms at baseline, 121 +/- 41 ms with the patch, and 135 +/- 26 ms after quitting. At 4 weeks after cessation of all nicotine use, the average heart rate remained higher, and HRV remained lower than values reported for healthy, middle-aged adults.