1. Isolated muscle spindles in the frog sartorius muscle were perfused in Ringer solutions made hypotonic by reducing NaCl or hypertonic by addition of NaCl, Na2SO4, Na2HPO4+NaH2PO4, sucrose, urea, glycerol, or propylene glycol.2. The frequency of spontaneous discharge at the in situ length of the muscle increased both in hypo- and hypertonic solutions.3. Both the dynamic and static components of the spindle potential during the stretch of the muscle increased in amplitude upon exposure to hypotonic solutions, while they decreased in hypertonic solutions.4. Dynamic and static responses of the spindle discharges during the muscle stretch increased in hypotonic solutions, while they decreased in hypertonic solutions.5. Changes in responses of the spindle receptor in the hypo- or hypertonic solutions were neither caused by changes in the steady potential along the sensory terminals nor by those in tension of the muscle fibers.6. The responses of spindle receptors in solutions with various osmolalities were discussed in terms of changes of ionic concentration in an interspace between the terminal axon and the surrounding Schwann process.