Calcineurin activation ameliorates the dystrophic pathology of hindlimb muscles in mdx mice and decreases their susceptibility to contraction damage. In mdx mice, the pathology of the diaphragm is more severe than in hindlimb muscles, and more representative of Duchenne muscular dystrophy. Constitutively active calcineurin-A was over-expressed in skeletal muscles of mdx dystrophic mice (mdx CnA*) to test whether muscle morphology and function would be improved. Contractile function of diaphragm strips and EDL and soleus muscles from adult mdx CnA * and mdx mice were examined in vitro. Hindlimb muscles from mdx CnA * mice had a prolonged twitch time course and were more resistant to fatigue. Due a slower phenotype and a decrease in fiber cross-sectional area, normalized force (sP_o) was lower in fast and slow hindlimb muscles of mdx CnA* than in mdx mice. In diaphragm, despite a slower phenotype and an ~35% reduction in fiber size, sP_o was preserved. This was likely mediated by the reduction in the area of diaphragm undergoing degeneration (i.e. mononuclear cell, connective, and adipose tissue infiltration). The proportion of centrally nucleated fibers was reduced in mdx CnA* compared with mdx mice, indicative of improved myofiber viability. In hindlimb muscles of mdx mice, calcineurin activation increased expression of markers of regeneration, particularly developmental myosin heavy chain (MyHC) isoform and MEF2a. Thus, activation of the calcineurin signal transduction pathway has potential to ameliorate the mdx pathophysiology especially in the diaphragm through its effects on muscle degeneration and regeneration and endurance capacity.