Plasticity in the cardiac function of a marine mammal facilitates rapid adjustments to the contrasting metabolic demands of breathing at the surface and diving during an extended apnea. By matching their heart rate (f _H ) to their immediate physiological needs, a marine mammal can improve its metabolic efficiency and maximize the proportion of time spent underwater. Respiratory sinus arrhythmia (RSA) is a known modulation of f _H that is driven by respiration and has been suggested to increase cardiorespiratory efficiency. To investigate the presence of RSA in cetaceans and the relationship between f _H , breathing rate (f _R ) and body mass (M _b ), we measured simultaneous f _H and f _R in five cetacean species in human care. We found that a higher f _R was associated with a higher mean instantaneous f _H (if _H ) and minimum if _H of the RSA. By contrast, f _H scaled inversely with M _b such that larger animals had lower mean and minimum if _H s of the RSA. There was a significant allometric relationship between maximum if _H of the RSA and M _b , but not f _R , which may indicate that this parameter is set by physical laws and not adjusted dynamically with physiological needs. RSA was significantly affected by f _R and was greatly reduced with small increases in f _R . Ultimately, these data show that surface f _H s of cetaceans are complex and the f _H patterns we observed are controlled by several factors. We suggest the importance of considering RSA when interpreting f _H measurements and particularly how f _R may drive f _H changes that are important for efficient gas exchange. This article is part of the theme issue 'Measuring physiology in free-living animals (Part I)'.