We tested the assumption that the slope of the within-individual regression equation of respiratory sinus arrhythmia (RSA) normalized by tidal volume (VT) upon respiratory cycle length (TTOT) can characterize ventilatory RSA modulation independent of cardiac vagal tone, whereas the intercept is varying as a function of vagal tone. We also explored whether a variation of VT is necessary to estimate slopes reliably. Four 3-min sequences paced at 8–18 cycles/min were performed supine and standing. Participants also breathed the same sequences in supine posture with voluntarily varying VT. The mean slope of RSA/VT upon TTOT was identical, and the mean intercept was lower for standing than supine (spontaneous VT) conditions. Stability of slopes was low between body postures, and was higher between spontaneous VT versus varying VT at the same body posture. The regression of RSA/VT upon TTOT allows for a valid estimation of ventilatory influences on RSA.