1. Periods of breath-holding are interrupted by episodes of continuous breathing in the aquatic turtle Pelomedusa subrufa, whereas single breaths and short periods of breath-holding alternate in the terrestrial tortoise Testudo pardalis. This implies that partial pressures of O2 and CO2 in expired air are stable in Testudo in contrast to cyclic fluctuations in Pelomedusa. 2. In spite of this, air convection requirements (VA/VO2, ml BTPS/ml STPD) are not significantly different for the two

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... s (25.4 in Testudo, 27.3 in Pelomedusa), and differences in weight-specific ventilation between the mean body weight. 3. The end-tidal PCO2 in Pelomedusa (mean 15.2 mmHg) is lower than in Testudo (mean 24.7 mmHg), which reflects aquatic CO2 elimination in Pelomedusa. 4. In Testudo, the time course of ventilation correlates with the time course of increase of end-tidal PCO2 during CO2 breathing, but no simple relationship is evident between ventilation and blood PCO2 in Pelomedusa. 5. In both species tidal volume as well as respiratory frequency increase approximately in proportion to the end-tidal PCO2, although the response to 6% CO2 breathing could be less than expected from extrapolation of the responses to 2 and 4% CO2. Both species also increase ventilation during hypoxia, but hyperoxia depresses ventilation.