Differences in contractile activation by Ca2+ and Sr2+ between various types of normal and dystrophic murine muscle fibres were investigated using mechanically skinned fibres derived from soleus and extensor digitorum longus (e.d.l.) muscles of normal and dystrophic mice of strain 129ReJ. In terms of contractile activation, the normal e.d.l. muscle was found to consist of one relatively homogeneous population of muscle fibres characterized by steep force-pCa and force-pSr curves, low sensitivity to Ca2+ and very low sensitivity to Sr2+. Normal soleus muscles contained two fibre populations of similar size which could be distinguished on the basis of their contractile activation properties. The first fibre population was characterized mainly by its shallow force-pCa and force-pSr curves, high Ca2+ sensitivity, high Sr2+ sensitivity and the occurrence of large, slow force oscillations of myofibrillar origin. The second fibre population was characterized by force-pCa and force-pSr curves of steepness intermediate between those of normal e.d.l. and those of the first fibre population of normal soleus, by faster myofibrillar force oscillations and by low sensitivity to Ca2+ and Sr2+. The dystrophic e.d.l. fibre population had contractile characteristics which were distinct from those of the three types of normal fibre populations. However, some characteristics of the dystrophic e.d.l. fibres were very similar to those of the normal e.d.l. fibre population. Of all the fibre types investigated, dystrophic e.d.l. fibres were the least sensitive to Ca2+. Dystrophic soleus muscle contained a single homogeneous population of fibres which shared some common contractile activation characteristics with both of the fibre populations present in normal soleus muscle. However, of all fibre types investigated, the dystrophic soleus fibres were the most sensitive to Ca2+. Because of this characteristic, these fibres formed a distinct population. The maximum tensions induced by Ca2+ and Sr2+ were usually smaller in dystrophic fibres than in normal fibres obtained equivalent muscles. In conclusion, various normal murine muscle fibre types can be identified on the basis of differences in the mechanism of force activation by Ca2+ and Sr2+. Furthermore, it is possible to detect significant physiological differences in the mechanism of force activation brought about by murine muscular dystrophy.