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1. Single muscle fibres from frog semitendinosus were subjected to sudden changes in [K]_o, while recording membrane potential.

2. In agreement with Hodgkin & Horowicz (1960), a sudden increase in [K]_o in normal fibres produced a rapid depolarization (half-time 0·3 sec), whereas a sudden decrease in [K]_o produced a slower repolarization (half-time 2-3 sec).

3. Fibres were subjected to `glycerol-treatment', a procedure which was supposed to produce a functional disconnexion of the T-system from the surface. In these glycerol-treated fibres both depolarization and repolarization induced by changes of [K]_o took place rapidly.

4. The results suggest that the slowness of the repolarization in normal fibres is due to a retention of K ions inside the T-tubules.

5. Electron microscopical observation of single fibres or bundles of fibres, which have been soaked in a Ringer containing ferritin, revealed that normal fibres contained ferritin particles in the T-system, while glycerol-treated fibres showed no ferritin. Except for the presence of some large vacuoles and some swelling of the T-system, glycerol-treated fibres appeared morphologically normal.

6. Prolonged soaking in a high potassium solution produced electrical effects suggesting that K ions can enter the tubules of treated fibres very slowly, in spite of their inaccessibility to ferritin.

7. The main effect of glycerol-treatment does not seem to be a total disconnexion of the T-system from the fibre surface, but rather constriction of the T-tubules near their openings to the exterior.

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