Changes in the volume of fiber water in hypotonic and hypertonic Ringer's solution were determined for the sartorius, stomach, and cardiac muscle of the frog using two methods. Loss of water in hypertonic solutions was nearly the same in all muscles, but swelling in hypotonic solutions was greatest in the sartorius, smallest in the heart. For the sartorius the deviation from the properties of an osmometer can be accounted for by a loss of electrolyte and by assuming that a small part of the fiber water is bound, but this appears insufficient to explain the behavior of stomach and cardiac muscle in hypotonic solutions. In very dilute solutions of CaCl(2) and MgCl(2) a large difference in concentration of electrolytes is maintained between the fibers and the medium. Under these conditions divalent cations, accumulating in the fibers, produce a change in physical properties which indicates increased internal cross-linking. It is suggested, therefore, that swelling is limited as in a gel and that a considerable hydrostatic pressure may develop within the fibers.