A study has been made of some optical and associated mechanical properties of resting frog's sartorius muscles in isotonic and hypertonic solutions. Tension and transparency changes accompanying small alterations of muscle length (smaller than 1-5%) were recorded simultaneously. 2. The form of the transparency change is complex. It has three phases, two of which occur during the length change and the third (delayed phase) after it is complete. Directional recording of the response reveals both scattering and diffraction components. 3. The change of light scattering is associated with tension changes in the short-range elastic component (SREC) of the muscle. Its magnitude is related to the stiffness of the SREC; both increase when the osmotic strength of the external solution is raised and when muscle length is increased. 4. The change of light scattering is not much affected by the velocity of the applied length change. This is true also of the mechanical stiffness of the SREC. 5. The origin of the diffraction change is not known. 6. It is concluded that the scattering effect is caused by conformational changes in the SREC.
