SYNOPSIS. Two groups of fishes, the tuna and the lamnid sharks, have evolved ounter-currentheat-exchange mechanisms for conserving metabolic heat and raising their body temperatures. Warm muscle can produce more power, and considering the other adaptations for fast swimming in these fish, it seems likely that the selective advantages of greater speed made possible by the warm muscle were important in the evolution of this system. Some tunas such as the yellowfin and skip jack are at a fixed temperature difference above the water, but bluefin tuna can thermoregulate. Telemetry experiments show that the bluefin tuna can maintain a constant deep body temperature during marked changes in the temperature of its environment.