1.  This study was performed to investigate a mechanism of heat exchange in sea turtles and how temperature and different acclimation time affects their metabolic rates.  In another part of this thesis I aimed to test the possibility of a correlation between dive duration and both metabolic rate and state of buoyancy known to be regulated via the gas volume in the lungs of Chelonian sea turtles. 2.  All experiments were conducted on captive loggerhead (<i>Caretta caretta) </i>and green turtles (<i>Chelonia mydas)</i> housed in a individual tanks with circulating seawater from the adjacent Gulf of Naples (Western Mediterranean). The total range of body masses of turtles used encompassed 2 to 60 kg. 3.  It was demonstrated, using Doppler ultrasound, that sea turtles change blood flow in their appendages in response to external cooling and heating. Although this was efficient to accelerate whole body warming and delay the cooling of the body, turtles eventually equilibrated their body temperatures with that of the surrounding water. 4.  The Q₁₀ effect on metabolic rate of sea turtles subject to acute exposure to varying temperatures was 1.3. However, during long term exposure to seasonally decreasing water temperatures turtles showed a more pronounced reducted of metabolic rate (O₁₀ = 5.4). Contemporaneously food intake and general activity were greatly reduced as well and dive durations increased.  Body temperatures showed the same seasonal trend as the decreasing water temperatures. 5. Oxygen consumption rates of individual turtles, measured over 24-h-periods, peaked at different times of the day and no specific dynamic action after feeding could be detected.