Introduction-Crystalline macromolecular systems which are axially oriented pos-

a specific action of these various reagents may differ considerably, melting of the tive crystalline polymer arrangement appears to be common to all processes of is nature.4 If melting has been carried to completion, spontaneous re-extension the fiber may be subsequently observed under conditions favorable to recrystalation only if the system contains a sufficient number of permanent intermolecular )sslinkages which survive the melting process. Pryor7 and Flory8'9 have suggested that muscular contraction is also the result of a melting or partial melting of axially oriented crystalline regions composed of lypeptide chains. The polymer chains comprising the muscle myofibrils are own to exist in the highly oriented crystalline state?0 so that the first requirement - contraction to occur by this mechanism is satisfied. The postulated change in ystallinity with contraction has not been satisfactorily demonstrated and the exriments reported herein were undertaken for this purpose. Specifically, the ange in length of glycerol-extracted rabbit psoas muscle upon immersion in ATP lutions of various concentrations at room temperature were inlvestigated. In ad;ion, the wide angle X-ray diffraction patterns at various stages of contraction ve been correlated with the dimensional changes. Experimental.--Glycerol-treated muscle fibers were prepared from rabbit psoas iscle following the method of Szent-Gyorgyi, 1 and stored at -20?C. Strips about irty to forty mm long and about 1 mm thick were washed thoroughly in water to nove the glycerol. The length in water was measured using a scale that could be id to 0.5 mm. The fiber was then immersed in a 0.1 M KC1 solution of the deed ATP concentration, for fifteen to thirty minutes. The fibers were periodically tnsferred to fresh solution during this period. After contraction in the ATP soluan, the specimens were returned to water and the new length was measured. The tio, in water, of the shrunken length to the original length was designated as L/Lo.