1. In both the turtle, Pseudemys scripta, and the tortoise, Testudo graeca, lung ventilation is closely accompanied by a tachycardia of predictable magnitude and duration. 2. Efferent vagal activity progressively decreases as heart rate increases with the onset of lung ventilation. Atropine decreases heart rate during apnoea to those levels observed during prolonged breathing series when the development, duration or magnitude of ventilation tachycardia. It is thus concluded that heart rate change during chelonian lung ventilation is mediated solely by alterations in vagal tone. 3. Peripheral sensory reflexes involving pulmonary stretch receptors, arterial chemoreceptors and baroreceptors, and receptors stimulated by water immersion do not affect heart rate during breathing. It is suggested that ventilation tachycardia in these chelonians is the result of the spread of activity between the respiratory and cardiac centres of the medulla.

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