The present study addressed the question of whether the slow fibres of mammalian skeletal muscle, containing the myosin heavy chain MHCI (type-I fibres), are a functionally homogeneous population. We compared various properties of Ca2+-activated, skinned, type-I fibres from the soleus and semitendinosus muscles of a rabbit. Soleus type-I fibres showed significantly faster kinetics of stretch activation, measured as the time-to-peak of the stretch-induced, delayed force increase, t3, than semitendinosus fibres (1239438ms, n=136, vs. 1600409ms, n=208 respectively) (meansSD, 22C). Similarly, the speed of unloaded shortening at 15C was faster in soleus than in semitendinosus fibres [0.790.16fibre lengths (FL) s1, n=44, vs. 0.650.15FL s1, n=35 respectively]. The kinetics of stretch activation were more temperature sensitive in semitendinosus than in soleus fibres. Finally, the generation of steady-state isometric force was more sensitive to Ca2+ in semitendinosus than in soleus fibres: [pCa50 (log [Ca2+] for half-maximal activation) at 22C: 6.290.15, n=28, vs. 6.190.10, n=18 respectively]. These results suggest strongly that there is no functional homogeneity within type-I fibres of different muscles. The observed differences might reflect the existence of more than one functionally different slow myosin heavy chain isoforms or other modifications of contractile proteins.