Respiratory sinus arrhythmia (RSA) or high-frequency heart rate variability has been widely used as a noninvasive measure of cardiac vagal tone. However, their dependency on both respiration rate and tidal volume is largely ignored. Only a minority of studies published in Psychosomatic Medicine in recent years has implemented precautions for controlling respiration rate in RSA measures, and tidal volume effects were only rarely addressed. We discuss methodologic issues related to respiratory control methods and present data that demonstrate that both respiration rate and tidal volume contribute substantially to the within-individual RSA variance under conditions of variable breathing, with tidal volume contributing up to one third beyond respiration rate. Finally, we outline a respiratory control method for the time-domain index of RSA and review research pertaining to its reliability, validity, and experimental application.