1. The mechanism of the enhancement and the deficit of isometric force by slow length changes in frog fast muscle fibres was studied by recording muscle fibre stiffness changes as measured with sinusoidal vibrations (0.5-1.9 kHz, peak-to-peak amplitude 0.1% of slack length, L0). 2. When a tetanized fibre was slowly stretched by 5-9% from sarcomere lengths 2.4-2.6 microns, the force rose to a peak during the stretch and then decreased towards a steady level higher than that during the ordinary isometric tetanus at the same sarcomere length. 3. The stiffness of the fibre first rose abruptly in response to stretch and then started to decrease linearly while the stretch went on; after the completion of stretch the stiffness decreased towards a steady value which was equal to that during the isometric tetanus at the same sarcomere length, indicating that the enhancement of isometric force is associated with decreased stiffness. 4. If a tetanized fibre was slowly released by 4-12% from sarcomere lengths 2.55-2.7 microns, the steady force attained after the completion of release was lower than that during an isometric tetanus at the same sarcomere length. 5. The stiffness of the fibre changed in parallel with the force both during and after the applied release. 6. Recordings of the segmental length changes along the fibre with a high-speed video system (200 frames/s) indicated that all segments lengthened in response to the applied stretch. 7. The segmental length changes in response to the applied release were markedly non-uniform; the length of a segment located at the centre of the fibre did not change appreciably both during and after the release. 8. These results are discussed in terms of cross-bridge performance and structure of the myofilament lattice.