The volume responses of the T-system of frog skeletal muscle to isotonic solutions of altered ionic composition, as studied by electron microscopic examination of osmium-fixed tissue, have been re-investigated using aldehyde fixatives.1. In confirmation of the earlier work we find that the T-tubes swell in frog sartorius muscles which have been soaked in Ringer solution in which the sodium chloride concentration has been reduced to 40 mM and tonicity maintained with sucrose before fixation in osmium.2. The swelling does not occur in muscles similarly treated, but fixed with acrolein or glutaraldehyde.3. Swelling of the T-tubes, which has been reported for osmium-fixed muscle following exposure to Ringer in which chloride is replaced by acetate, or reduced and sulphate substituted, does not occur when fixation is by acrolein.4. Analysis of the data from the earlier work with sucrose substitution shows that the degree of T-tube swelling is proportional to the sucrose concentration of the soaking medium, whether the solution be isotonic or anisotonic.5. It is concluded that the increase in T-tube volume arises in muscles which have been exposed to certain impermeant solutes during fixation by osmium, and a possible mechanism to account for the effect is described.6. The relevance of these observations to the hypothesis that the sarcoplasmic reticulum is an extracellular compartment in muscle and to the proposal that the T-tubes may represent an intermediate compartment for potassium fluxes is discussed.