Chemically skinned fibres from soleus and plantaris rat muscles were used to compare the contractile properties of slow and fast muscles. The maximal isometric tension appeared larger in plantaris than in soleus fibres. The apparent Ca z + threshold for activation was lower in slow than in fast fibres while Ca 2 + concentrations required to obtain either the maximal tension or half maximal tension (pCas0) were lower in fast than in slow fibres. This apparent difference in Ca 2 + sensitivity will be discussed. As could be expected from other studies, a faster force development in plantaris than in soleus fibres occurred. However, one interesting new result showed that in soleus, the kinetics of the tension development estimated by the tmax parameter were slightly dependent on the Ca z + concentration whereas the ts0 parameter changed significantly with the Ca 2 + concentration. In plantaris, both tmax and tso parameters were found to depend strongly on the Ca 2+ concentration. Finally, the plantaris muscle showed a greater caffeine sensitivity than the soleus muscle. All the results suggested that the Ca-regulatory mechanism in the slow fibres was essentially different from that in the fast fibres.