Tension transients were recorded at sarcomere lengths from 2.0 to 3.2 mum in isolated fibres from the tibialis anterior muscle of frogs during tetanic stimulation at 0-1 degrees C. 2. The length of a selected portion of the fibre was controlled by feed-back from a spot-follower device. The step was complete in 0.2 ms and the natural frequency of the force transducer was 10.8 kHz. 3. The transients were analysed by comparing the tension record with the output of an analogue circuit (delay line) which contained components representing (a) force transducer response, (b) fibre inertia, (c) viscosity and inertia of surrounding fluid, (d) passive stiffness and viscosity of the fibre, (e) tendon compliance and (f) stiffness and early tension recovery of the contractile apparatus. 4. In releases at different sarcomere lengths, the instantaneous stiffness and the early tension recovery attributed to the contractile apparatus varied almost exactly in proportion to the developed tension. In the later phases of the transient there were minor deviations from proportionality. 5. The results confirm that the entire transient represents events in the cross-bridges. 6. At full overlap, the compliance attributable to the cross-bridges is at least 80%, and probably well over 90% of the measured instantaneous compliance of the fibre. Stiffness can therefore be used as a measure of the number of attached cross-bridges. 7. The amount of instantaneous sliding movement of thick relative to thin filaments required to bring tension in a cross-bridge from the isometric value to zero is about 3.9 nm if filament and Z-line compliance are negligible, as suggested by the results. It is not however excluded that filament compliance, though small, may be sufficient to reduce this figure to 3.5 nm or possibly 3.1 nm. 8. The responses to quick stretch, unlike those to release, could not be satisfactorily matched with the delay line. The deviations suggest that the instantaneous elasticity is non-linear in stretches. 9. In resting fibres at all sarcomere lengths, the first peak of the tension response was determined chiefly by fibre inertia and viscosity, rather than elasticity.
