Heart rate (FH) accelerates in inspiration and decelerates in expiration, a phenomenon known as Respiratory Sinus Arrhythmia (RSA). Although the presence of RSA has been documented in many species, how its magnitude compares among species is unknown. We asked whether the magnitude of RSA in a large mammal, the horse, differed from that of previously measured humans. From electrocardiogram and pneumography, the peaks and troughs of FH were identified breath-by-breath in four horses (Italian Saddlebred geldings) during resting wakefulness. RSA was computed as the peak-trough FH difference, in percent of mean FH. Horses had lower FH and respiratory frequency (FR) than humans, but similar FH/FR. RSA ranged between 6% and 15%, with an average of 9 ± 2%, not statistically different from the mean value in humans (12 ± 1%). Like in humans, in horses the FH/FR values below the mean had correspondingly lower RSA, while values above the mean had correspondingly higher RSA. If confirmed in other species, these results suggest that RSA is body size-independent. The correlation with FH/FR, rather than FH or FR, supports the view that RSA optimizes the coupling between pulmonary blood flow and ventilation.