1. Force responses to ramp stretches were recorded in single muscle fibres isolated from the lumbricalis muscle of the frog. Stretches were applied at rest and at progressively increasing times after a single stimulus. 2. The increase of fibre stiffness that precedes tension development has a 'static' component that accounts for the whole fibre stiffness increase during the latent period and at very low tension at the beginning of the twitch. 3. Static stiffness increase was not affected by 2,3-butanedione-2-monoxime, a drug that almost completely inhibited twitch tension. 4. Static stiffness increased approximately 5-fold as the sarcomere length was increased from 2.1 to 2.84 microns. 5. These results suggest that static fibre stiffness increase is not attributable to the formation of non-force-generating cross-bridges.