Abstract-1. In keeping with other reptiles, core temperature of the lizard Physignathus lesueurii responds more rapidly to a step function increase in temperature than to a corresponding decrease.2. Observations on twelve species (five families) of non-Chelonian reptiles heating and cooling in air and water show that strongly predictable relationships exist between thermal time constants and body size. Chelonia show a different pattern.3. These observations are compared with the predictions of a simple model which, although not sufficiently complex to simulate physiological changes, provides insight into the relative importance of the physical and biological factors which underlie the observed relationships.

SUMMARY1. In unanaesthetized sheep, the sensitivity of the baroreceptor-cardio-inhibitory reflex was greater when intravenous vasopressin was used to raise blood pressure, than when intravenous phenylephrine was used to raise blood pressure. This difference was still evident in animals in which fl-adrenergic blockade had been carried out using propranolol.2. In the presence of combined f-adrenergic and muscarinic blockade, a direct negative chronotropic effect of intravenous vasopressin could not be demonstrated. It was concluded, therefore, that intravenous vasopressin enhanced cardiac vagal tone.3. This effect of vasopressin on efferent cardiac vagal tone was confirmed directly in anaesthetized dogs by recording from single cardiac vagal efferent fibres. Furthermore, recordings from single carotid sinus baroreceptor fibres did not demonstrate a direct action of vasopressin on the sensitivity of the baroreceptors. However, the pressor effect of vasopressin is associated with a greater increase in efferent cardiac vagal discharge than that seen when equipressor doses of phenylephrine are given, or when blood pressure is raised by a similar amount by inflation of an intra-aortic balloon.4. Studies in isolated guinea-pig atrial preparations and in anaesthetized rabbits and dogs, revealed no consistent peripheral action of vasopressin on the action of the vagus at the heart.

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