Ferritin and Imferon molecules were introduced as tracers inside "skinned" muscle fibers to test which part of the triadic junction gap is freely exchangeable with the sarcoplasm. At least 50% of the T-system surface is freely accessible from the sarcoplasm. Of the remainder, 30% of the total T-system surface is covered by the junctional feet, and 20% in the center of the junction may or may not be accessible. The possibility is discussed that the triadic junction may not function as an electrical coupling.