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The fine structural differences of motor endplate among the red, white and intermediate muscle fibers of the rat fast limb muscle, M. extensor digitorum longus, were observed by the electron microscope.The motor endplate of the red fiber was small. The junctional folds were poorly developed and their number was small. In the white fiber, the motor endplate was large and the junctional folds were well developed and their number was far more numerous than those in the red fiber. In the intermediate fiber, the development of junctional folds of motor endplate was rather similar to that of the white fiber although the number of the junctional folds was a little less than that of the white fiber.

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An electron microscopic study was made on the fine structural differences in the motor endplates among the red, white and intermediate muscle fibers of the human intercostal muscles and the following results were obtained: 1) In the motor endplate of the red fiber, the junctional folds appear on the whole as simple comparatively unbranched folds, while in that of the white one they are branched and anastomosed in a complicated manner. In the motor endplate of the intermediate fiber, the character of the folds is intermediate between those of the red and white fibers.2) The sarcoplasmic area where the axonbranchlet is situated is apparently narrower in the red fiber than in the white or intermediate fibers.3) The terminal branchlet in the red fiber is oval in shape but in the white one it has a concavity oriented towards the underlying muscle fiber. In the intermediate fiber the branchlet is intermediate between those of the red and white fibers.

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