Because tunas possess countercurrent vascular pathways serving the trunk musculature, metabolic heat is retained, and muscle temperatures can considerably exceed that of the surrounding water (+1° to +21°C). And because tunas have this excess, it is reasonable to suppose they have some means of controlling its magnitude. Tunas must contend with two exigencies which can perturb body temperature: changes in water temperature and, in contrast to non-thermoconserving fish, changes in activity. Both can be met by adaptive change in excess muscle temperature. If this could be accomplished in the absence of changes in environmental temperature or activity level, this would constitute physiological thermoregulation. If excess muscle temperature cannot be altered sufficiently to acceptable levels, more favorable environmental temperatures must be sought or activity levels changed. We would consider this behavioral thermoregulation. High sustained swim speeds, characteristic of the continuously swimming tunas, require special consideration. Heat production is proportional to approximately the cube of swim speed. In order to maintain a slight temperature excess at basal swim speeds (1–2 lengths/sec), and yet not overheat during sustained high speed swimming (>4 lengths/sec), mechanisms are required to conserve heat under the former conditions and to dissipate it effectively under the latter. In this report, we review published observations other investigators have interpreted as physiological thermoregulation in tunas, describe recent findings in our laboratory, and suggest some possible thermoregulatory mechanisms.