The influence of [Ca2+] on the maximum velocity of shortening (Vmax) was examined in mechanically skinned Rana pipiens and Rana temporaria fibres using improved force clamps and the slack test techniques. All measurements were made at 7.5 degrees C. At low relative loads (P/P0 less than 0.1), maximally activated R. pipiens fibres shortened more rapidly than did submaximally activated fibres. At higher relative loads, however, little difference in the speed of shortening was observed. Vmax (determined by the slack test) of R. pipiens fibres increased as the level of activation increased. Over sarcomere lengths 1.8-2.1 microns it was 2.28 muscle lengths/s (m.l./s) (S.E. of mean +/- 0.25, n = 5) at 20-35% activation, 2.89 m.l./s (+/- 0.22, n = 7) at 40-60% activation, and 4.18 m.l./s (+/- 0.25, n = 6) at 100% activation. At longer sarcomere lengths (2.2-2.6 microns), higher Vmax values were observed at all levels of activation, but the influence of Ca2+ on Vmax persisted. Vmax was 3.54 m.l./s (+/- 0.41, n = 4) at 20-30% activation and 5.15 m.l./s (+/- 0.22, n = 5) at 100% activation. In R. temporaria fibres, Vmax (determined by force clamps over sarcomere lengths 1.8-2.1 micron) also increased as the level of activation increased, from 3.47 m.l./s (+/- 0.06, n = 6) at 13-29% activation to 5.62 m.l./s (+/- 0.17, n = 6) at 100% activation. Vmax was also determined (using the slack test) in mechanically and chemically skinned rabbit soleus fibres. Vmax at 15 degrees C (1.05 m.l./s, +/- 0.11, n = 5) at full activation decreased by more than 3-fold as the level of activation was reduced to 10%. We conclude that the level of activation influences the Vmax of skinned skeletal muscle fibres. This has now been demonstrated in three different preparations and by a variety of techniques. This effect is most pronounced at low relative loads, and might not be observed if there are experimental limitations which prevent making velocity measurements at low relative loads.