Ambulatory respiratory sinus arrhythmia (RSA), or high-frequency heart-rate variability (HRV), is frequently employed as an index of cardiac parasympathetic control and related to risk or severity of cardiovascular disease. However, laboratory studies indicate variations in physical activity and respiratory parameters of rate and tidal volume may confound estimation of vagal activity. Because little is known about these relations outside the laboratory, we examined ambulatory relations between RSA, respiration, physical activity and heart rate (HR) during waking hours, employing a multi-channel monitoring system. Forty healthy young-to-middle aged adults underwent daytime monitoring that included continuous registration of the ECG, respiration (inductance plethysmography) and accelerometry motion activity. Within-individual regression analyses were performed to examine minute-to-minute relations between RSA and respiration, HR, and indices of physical activity (minute ventilation and motion). HR changes were assumed to be strongly related to within-individual variations of vagal tone. RSA adjusted for respiratory parameters and unadjusted RSA were compared for strength of prediction of other measures. Unadjusted RSA was related to respiratory parameters (R= 0.80), and moderately predicted minute-tominute HR and activity variances (means = 56%, HR; 48%, minute ventilation; and 37%, motion). Adjusted RSA predicted significantly more HR and activity variance (means = 75%, 76% and 57%, respectively) with narrower confidence intervals. We conclude that ambulatory RSA magnitude is associated with respiratory variations and physical activity. Adjustment for respiratory parameters substantially improves relations between RSA, and significantly vagally mediated HR and physical activity. Concurrent monitoring of respiration and physical activity may enhance HRV accuracy to predict autonomic control.