Respiratory sinus arrhythmia was measured on 24 male subjects whose ages ranged from 21 to 65 yr, using a technique in which respiration was coupled to heart rate, so that there were 6 heartbeats per inspiration and 6 heartbeats per expiration. This voluntary cardiorespiratory coupling reduced the variance of heart rates measured at various points in the respiratory cycle. Fourier analysis was used to obtain the fundamental of heart rate and respiratory volume. A high correlation coefficient (-0.83) was found between percent of variation of heart rate from the average heart rate (PB) and the age of the subject. Linear regression analysis performed on these data produced the equation: PB = 23.2-0.35 (age). Since sinus arrhythmia at the low respiratory frequencies used probably reflects the viability of the cardiorespiratory control systems, we believe this provides a simple noninvasive method for the study of the overall competence of the system responsible for respiratory sinus arrhythmia.