The velocity of 'unloaded' shortening (V0) and the force-velocity relation were studied during fused tetani (0.5-2.0 degrees C) in short successive segments along the entire length of single fibres isolated from the tibialis anterior muscle of Rana temporaria. The segments were defined by opaque markers of hair that were placed on the fibre surface, 0.5-0.8 mm apart, from one tendon insertion to the other. The change in distance between two adjacent markers (one segment) was monitored by means of a photoelectric recording system, while the fibre was released to shorten isotonically between 2.2 and 2.0 micron sarcomere lengths. The accuracy of the V0 measurement was better than 4% in all parts of the fibre. V0 varied along the length of the fibre, each fibre having a unique velocity pattern that remained constant throughout the experiment. The difference between the highest and lowest values of V0 within the fibre varied between 11 and 45% of the fibre mean in thirty-two preparations (mean difference 23 +/- 2%, S.E. of mean). An attempt was made to relate the V0 pattern to the fibre's orientation in the body in fourteen complete experiments. The highest values of V0 were obtained near the proximal end of the fibre, and there was a clear trend for V0 to assume lower values towards the distal end. The V0 pattern along the fibre did not correlate with the segments' capacities to produce force nor with the passive viscoelastic properties of the segments. Force-velocity data obtained from individual segments provided a good fit to Hill's (1938) hyperbolic equation at loads less than 80% of the measured tetanic force. The curvature of the force-velocity relation, defined by alpha/P0 in Hill's equation (P0 being the isometric force calculated from the hyperbolic function) varied between 0.09 and 0.46 in sixteen segments of six different fibres. V0 was inversely related to alpha/P0 according to the following regression: V0 = 3.21 - 3.22. (alpha/P0), correlation coefficient, 0.72; P less than 0.005. No clear correlation between V0 and alpha/P0 existed at the whole-fibre level. The results support the view that the kinetic properties of the myofilament system differ from one region to another along the length of a muscle fibre.