1. The measured extracellular space of the taenia coli is large when small ions or molecules are used for the determination, and small when large molecules are used, even with identical experimental procedures.2. Extracellular hyaluronic acid has been detected histologically. It is apparently reduced by hyaluronidase.3. The extracellular inulin space increases after the tissue has been pretreated with hyaluronidase, although the ionic composition and wet weight are unchanged.4. It is suggested that the hyaluronic acid prevents the free entry of macromolecules such as inulin into the extracellular space by steric hindrance. Monatomic ions such as Na(+), Li(+), Cl(-), Ca(2+) therefore have a larger extracellular space available than is calculated on the basis of the inulin space.5. A slight shrinkage of the muscle cells can be detected when the incubation period is prolonged.