We investigated the calcium sensitivity for tension generation of different fibre types and the possible correlation between calcium sensitivity and the presence of distinct regulatory protein and myosin light chain (MLC) isoforms in rat skinned skeletal muscle fibres. Fibre types 1, 2A and 2B were identified by electrophoretic analysis of myosin hea~?' chain (MHC) isoforms. Fibres showing more than one MHC isoform were discarded. Type I fibres from the soleus showed a higher pCa , where [ ] denotes concentration) threshold and a lower slope of pCa/tension CUlWe than type 2 extensor digitorum longus (EDL) fibres; between type 2 fibres, type 2 B showed the higher slope of pCa/tension curve. Type I fibres from different muscles showed similar calcium sensitivities when containing only the slow set of regulatory proteins and MLC; when both slow and fast isoforms were present, calcium sensitivity shifted toward fast type fibre values. Type 2A fibres from different muscles showed a similar calcium sensitivity, independently of the set (purely fast or mixed) of regulatory proteins and MLC. It is suggested that when both fast and slow isoforms ofregulatory proteins and of MLC are present in a muscle fibre, calcium sensitivity is dictated mainly by the fast isoforms.