1. The relationship between force and velocity of shortening was studied during fused tetani of single fibres isolated from the anterior tibialis muscle of Rana temporaria (1.5-3.3 degrees C; sarcomere length, 2.20 microns). Stiffness was measured as the change in force that occurred in response to a 4 kHz length oscillation of the fibre. 2. The results confirmed the existence of two distinct curvatures of the force-velocity relationship located on either side of a breakpoint in the high-force, low-velocity range. Reduction of the isometric force (P0) to 83.4 +/- 1.7% (mean +/- S.E.M., n = 5) of the control value by dantrolene did not affect the relative shape of the force-velocity relationship. The breakpoint between the two curvatures was located at 75.9 +/- 0.9% of P0 and 11.4 +/- 0.6% of maximum velocity of shortening (Vmax) in control Ringer solution and at 75.6 +/- 0.7% of P0 and 12.2 +/- 0.7% of Vmax in the presence of dantrolene. These results provide evidence that the transition between the two curvatures of the force-velocity relationship is primarily related to the speed of shortening, not to the actual force within the fibre. 3. The instantaneous stiffness varied with the speed of shortening forming a biphasic relationship with a breakpoint near 0.15 Vmax and 0.8 P0, respectively. The force/stiffness ratio (probably reflecting the average force per cross-bridge), increased with force during shortening. The increase of the force/stiffness ratio with force was less steep at forces exceeding 0.8 P0 than below this point. 4. A four-state cross-bridge model (described in the Appendix) was used to evaluate the experimental results. The model reproduces with great precision the characteristic features of the force-stiffness-velocity relationships recorded in intact muscle fibres.