Intracellular recordings of the negative after-potential that follows a train of impulses in frog muscle are described. It is postulated that the cause of this after-potential is an accumulation of potassium, during the train, in an intermediary space that is located between the major portion of the sarcoplasm and the external fluid. The decline of the after-potential is then produced by the exchange of potassium from the intermediary space with sodium from the external fluid.

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