The degree of parasympathetic control of resting heart rate (PC) was assessed by measurement of variation in heart period (VHP) during cardiopulmonary synchronization of respiration. Respiratory period was arbitrarily preset and standardized at 7 heart beats (3 beats inspiration, 4 beats expiration). The mathematical and experimental evidence for this technique is elucidated. Twenty-one healthy subjects were examined for the relationship of aerobic power (VO2max) to (1) VHP and (2) respiratory sinus arrhythmia (RSA) waveform amplitude and phase. Intraindividual variability in VHP was low (test-retest r = 0.97-0.98). VHP in msec was a logarithmic function of VO2max (ml X kg FFW-1 X min-1) according to the equation: 1n VHP = 0.27 + 0.082 X VO2max where r = 0.92, P less than 0.001). VHP was more closely related to VO2max than was resting HR (r = -0.75). In addition, a higher VO2max was associated with a leftward shift of the HR: respiratory cycle sinusoidal curve. These results illustrate the close relationship between aerobic power and vagal tone in control of resting heart rate. Individuals with a higher aerobic power maintain lower resting heart rates mainly via an increase in parasympathetic tone (as opposed to a decreased sympathetic tone). Previous correlations between RSA characteristics and age may be accounted for by age-related decreases in VO2max.