An electron microscope study is reported of structural changes during ATP-induced contraction of glycerinated rabbit psoas. In the absence of ATP, A-band length is constant at sarcomere lengths above 1.9 micron, with average length of 1.54 mu. In ATP-treated fibers, A-band length is also constant at sarcomere lengths above 2.0 microns, but the apparent length of A-band decreases to approximately 1.3 micron, as sarcomere length decreases from 1.9 micron to 1.5 mu. The occurrence of short A-bands cannot be attributed to crumpling of thick filaments against Z-lines, since I-bands remain patent; nor to the presence of heterogeneous filaments, since resting muscle does not show comparable heterogeneity, nor to compressive artifacts, which are minor when knife edge is oriented parallel with fiber axis during microtomy . The decrease of A-band length appears related, at least in part, to disarray of terminal cross-bridges as the thick filaments encroach upon the N-line, a structure which becomes evident within the I-band during contraction of glycerinated fibers. In preliminary studies, optical transforms of A-bands from individual sarcomeres reveal a characteristic myosin layer-line pattern as low as 1.5 micron sarcomere length. A cross-bridge repeat of 143 A is obtained for sarcomeres above 1.6 micron length; however, an appreciable proportion of sarcomeres in the range from 1.5 micron to 1.9 mu length generate meridional reflections less than 143 A, and as low as 130 A.