1. A series of breathing movements which are effective in ventilating the lungs are accompanied by a marked increase in pulmonary blood flow and a decrease in pulmocutaneous arterial pressure. This reaction must involve considerable vasodilation of the pulmonary vascular bed.

2. Similar vasodilation is produced by artificial inflation of the lungs via an implanted cannula. Nitrogen is not so effective as air in producing the vasodilation, whereas oxygen is more effective. It is suggested that both stretch receptors in the lungs and chemical receptors in lungs or blood are involved in the reaction.

3. The level of anaesthesia is important in determining the degree of vasodilation response to the different stimuli. It is concluded that the central nervous system is the site where interaction occurs between signals from stretch receptors and chemical receptors and from the breathing movements themselves. Experiments with atropine suggest that the efferent pathway is in the vagus nerve.

4. Recordings from free-swimming, unanaesthetized toads show that the vasodilation response occurs as part of normal diving-emergence behaviour.

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