The mechanical and energetic properties of extensor digitorum longus (EDL) and soleus muscles of X chromosome-linked muscular dystrophic mutant (mdx) mice aged 4-6 weeks were studied and compared with those of the muscles of normal mice. Maximum tetanic tension, the speed of contraction of relaxation, and the heat production of mdx soleus muscles were not significantly different from those of the normal muscles. However, in mdx EDL muscles, the tension and heat production were significantly reduced, and relaxation was prolonged. To study the cause of these changes in mdx EDL muscles, tension and heat production were measured at various muscle lengths greater than optimum for tension. Both the amount of twitch heat and the heat rate for a tetanus were linearly related to the tension and had non-zero intercepts at zero tension, the activation heat. The twitch activation heat and the tension-related heat in tetani of mdx EDL muscles were not different from those in normal muscles. On the other hand, the tetanus activation heat of mdx EDL muscles was significantly smaller than that of normal muscles. Assuming that the degenerated fibers do not contribute to the active force produced, these results suggest that the amount of Ca2+ released in a contraction is not significantly different between normal and mdx muscles, but the Ca-ATPase activity of the salcoplasmic reticulum is reduced in mdx EDL, which could cause the slowing of relaxation.