Diving bradycardia, or a decrease in heart rate while diving, has been observed in a wide variety of species, from frogs to seals. The degree to which an animal lowers its heart rate during a submergence may vary depending on the behaviour of the animal. The leatherback sea turtle (Dermochelys coriaced) is an active diver, and has a metabolic rate that is higher than expected for a reptile of its size. The purpose of this research was to gain an understanding of the cardiovascular response to diving made by the leatherback turtle and to integrate this information with what is known of the turtle's diving behaviour and metabolism. Instruments capable of recording heart rate, dive duration and dive depth were deployed on leatherback females as they lay eggs on a nesting beach. Heart rate and diving variables were recorded during the internesting interval when turtles were freely diving at sea. Instruments were recovered when the turtles returned to the nesting beach to lay more eggs. Relationships between the turtle's diving behaviour and heart rate were examined, and comparisons were made between diving heart rates, surface heart rates, and long-term heart rates. Dive records show that leatherback turtles dive continually throughout the internesting interval. The brief amount of time spent at the surface between dives suggests that these turtles rely mainly on aerobic metabolism for routine dives, and all observed dives fell within the calculated aerobic dive limit. No significant diel differences existed for leatherback turtles from the Playa Grande nesting population, but dives became shorter and shallower as the internesting interval drew to a close. Neither dive duration nor dive depth was strongly related to heart rate. Diving heart rates were significantly lower than surface heart rates for leatherback turtles. The most likely cause of the observed diving bradycardia is increased pulmonary resistance accompanied by variable degrees of systemic perfusion. The fact that decreases in heart rate during diving were not extreme suggests that turtles do not drastically lower their metabolic rate while diving during the internesting interval. Long-term heart rates were more similar to diving heart rates than to surface heart rates.