Abstract The primary oxygen stores in Cheloniidae sea turtles are in the lungs. Therefore, management of blood oxygen transportation to peripheral tissues by cardiovascular adjustments while diving is crucial to maximize benefits from dives. However, heart rate, particularly cardiac response to exercise in free-ranging dives, has rarely been examined for sea turtles. In this study, heart rate and its relationship with the amount of activity were determined in six free-ranging green turtles using bio-logging techniques. Our results demonstrated that resting heart rate took 7–11 h to reduce to steady levels after turtles were released in the tank, indicating that turtles may not present normal physiological rates right after release. After heart rate reduction, resting heart rate of green turtles in free-ranging dives was generally low (10.9 ± 2.5 bpm), but they often presented arrhythmia (4–54 bpm) even in resting states. The amount of activity during a dive linearly increased heart rate, but maximum heart rates (39.0–69.8 bpm) were recorded during ventilation at surface. These results indicate that turtles have the capability of cardiac response to increased metabolic demands in their muscles while submerged, and also of cardiovascular adjustment for a rapid renewal of oxygen stores and removal of CO 2 during ventilation. Such well-organized cardiac adjustments may be because of characteristics of Cheloniidae sea turtles such as ectothermy and oxygen storage in lungs while submerged. Summary statement Green sea turtles in free-ranging dive had generally lower heart rate compared to other air-breathing divers and it varied with the amount of exercise. Turtles often showed extreme arrhythmia.