The purpose of this investigation was to examine the effect on duration of vagal asystole caused by changing the oxygen and carbon dioxide content of arterial blood. The pH of blood was determined in every experiment. It was proved that, in control animals, the duration of vagal asystole varied from dog to dog, but it was constant in each animal receiving repeated stimulation under the same conditions. A decrease in O2 content or an increase in CO2 content of arterial blood, or both, produced a longer period of asystole. The possible mechanisms are discussed.

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