We appreciate the interest by Drs. Vaschillo and Lehrer in our Opinions/Hypothesis article (February 2004)1on respiratory sinus arrhythmia (RSA). We agree with their comment that heartbeat synchronizes with respiratory rhythm only under certain conditions, so that the results of the physiologic experiment are only applicable to the model used in the study. In their well-organized investigation,2 Vaschillo and coworkers2 used healthy subjects who were strictly instructed to breathe in synchrony with the extrinsic pacemaker (ie, a metronome) at predetermined respiratory frequencies between 0.04 and 0.5 Hz for 2 min. As was noted in our Opinions/Hypothesis article,,1 humans are a species with a weak RSA compared with dogs. Moreover, a paced breathing that is in synchrony with the extrinsic rhythm generator might precipitate mental stress for subjects, and the equilibrium state of CO2/O2 metabolism through the paced breathing could not have been obtained in a very short period of time. Such factors as species, state of the subjects (ie, very alert, alert, relaxed, or asleep), and metabolism should always be considered in clinical/basic experiments on the synchrony of heartbeat with respiratory rhythm. Therefore, for this purpose we used trained dogs to lie down in a relaxed state under spontaneous breathing,,3–5 as dogs are a species with a strong RSA.