1. The possibility of retarding the diffusional processes in the transverse tubules of frog single muscle fibres was explored by increasing the viscosity of the bathing medium. The viscosity was increased by adding Dextran 15 to the experimental solutions.2. The criterion used to test this possibility was the speed of repolarization of muscle fibre membranes produced by sudden reduction in [K](o) in media of different viscosity.3. In fibres equilibrated in media containing 15% Dextran 15, and that had been depolarized with 80 mM-K for more than 5 sec, the repolarization caused by lowering the [K](o) to 40 mM proceeded more slowly than in fibres in Dextran-free media.4. On the basis of the above results, it was deduced that high viscosity solutions would delay the diffusion of Na ions in the transverse tubules. Thus the effect of sudden changes in [Na](o) on the twitch tension of single fibres could be studied.5. The recovery of twitches when the fibres were exposed from a solution containing no Na, to one that contained 46 mM-Na, occurred more slowly in high viscosity media.6. This result can be explained assuming that a Na regenerative process occurs at the level of the membrane of the transverse tubules.