Green sea turtles (Chelonia mydas) are well known as endurance swimmers and divers. Physiological correlates of these traits were studied in 9 adult sea turtles (mean body mass=87 kg) at a body temperature of 25°C. The respiratory properties of the blood were similar to those of other turtles except for a higher oxygen affinity (P50=18.2 Torr, pH 7.6), which may be an allometric function. Resting, systemic blood flow, calculated from the Fick principle was 21.5 ml·kg−1. min−1, similar to values reported for other turtles. Pulmonary blood flow, measured by mass spectrometry of acetylene uptake in the lungs was 24.0 ml·kg−1·min−1, not significantly different from the calculated systemic flow. Other evidence of a small (net) intracardiac shunt is the high arterial saturation (ca. 90%) of arterial blood. This distinctive feature of O2 transport inC. mydas provides an\([Ca_{O_2 } - C\bar \upsilon _{O_2 } ]\) content difference of 4.1 ml· dl−1. This results in a relatively low blood convection requirement at rest\(\dot Q_t /\dot V_{O_2 }\)=24.4 mlbtps·mlstpd−1), similar to that for many mammals. This would favor a high maximum O2 uptake, as measured by others in this species. The relatively high O2 affinity of blood in this species could be adaptive to “loading” O2 during intermittent breathing while swimming and to utilizing the lung O2 store during the progressive hypoxia of diving.