In this work described experiments to show the influence of the pH on convulsions caused by cooling the spinal cord of frogs, and to study the mechanism of the inhibitory action of carbon dioxide on these convulsions. We found that carbon dioxide does not act by lowering the pH of the blood or intercellular medium. Carbon dioxide would have a specific action which would be connected with its great diffusibility. This property would enable it to penetrate rapidly into the interior of the neurone, appreciably lowering the intracellular pH, in the absence of a clear variation in the pH of the blood.