The contractile properties of soleus muscles from mdx and control mice aged between 26 and 350 days were compared with those of muscles from similarly aged control mice. Mdx mice were in general heavier (their individual soleus muscles were also heavier), of greater cross-sectional area and greater standard length than age-matched controls. Isometric forces produced by soleus muscles from young mdx mice (less than or equal to 100 days) were similar to controls, but were weaker when force was normalized for cross-sectional area. Conversely, although the absolute isometric forces produced by older (greater than 100 days) mdx muscles were greater than age-matched controls, when normalized for cross-sectional area they were similar. No differences were found between mdx and control muscles in terms of length-force or force-velocity relationships. Thus, young mdx control muscles produce similar absolute isometric force but mdx mouse muscles are larger. When muscle size is accounted for, in terms of cross-sectional area, younger mdx muscles are, therefore, weaker than controls. Inefficient contraction of young mdx muscles may result from lack of contractile fibres, physiological inefficiency of contractile fibres, or loss of tendon-fibre continuity during muscle fibre necrosis and regeneration. The striking supernormal size and strength of older mdx muscles reflects their considerable regenerative capacity; whether this is due to an increase in muscle fibre number rather than fibre hypertrophy remains unclear.