1. To explore how maximum velocity of shortening (Vmax) of fibres varies within one muscle and how Vmax varies with body size, we measured Vmax of muscle fibres from soleus muscle of a large animal, the horse. 2. Vmax was determined by the slack test on skinned single muscle fibres at 15 degrees C during maximal activation (pCa = 5.2). The fibre type was subsequently determined by a combination of single-cell histochemistry and gel electrophoresis of the myosin light chains. 3. Vmax values for the type I, IIA and IIB muscle fibres were 0.33 +/- 0.04 muscle lengths/s (ML/s) (+/- S.E.M., n = 6), 1.33 +/- 0.08 ML/s (n = 7) and 3.20 +/- 0.26 ML/s (n = 6), respectively. It is likely that the large range in Vmax is due to differences observed in the myosin heavy chains and light chains associated with the three fibre types. 4. Comparison of Vmax over a 1200-fold range (450 kg horse vs. 0.38 kg rat) of body mass (Mb) suggests that slow fibres scale more dramatically (Mb-0.18) than do fast glycolytic fibres (Mb-0.07). This difference may enable the slow fibres to work at high efficiencies in the large animal while the fast fibres can still generate a large mechanical power when necessary.