Skinned fibres from frog semitendinosus muscle were stretched in relaxing solution from a sarcomere length of 2.5 microns to greater sarcomere lengths, and then shortened back to the original length. Fibres could be stretched up to sarcomere lengths of 3.3 microns, and reshortened fully. If the original stretch was to a sarcomere length greater than 3.3 microns, the extent of recovery was dependent on the magnitude of the stretch and the number of times the stretch/shorten cycle was repeated. When the original stretch was to sarcomere lengths beyond overlap of the thick and thin filaments, the thin filaments did not re-enter the thick filament array but buckled at the A-I junction. If these fibres were subsequently activated and contracted, the thin filaments re-entered the thick filament array, taking up approximately their former positions, and allowing reduced development of isometric tension.