1. Extensor digitorum longus muscles of C57 BL/10 and mdx mice were overloaded by removing the synergist tibialis anterior muscle of 9–12-day-old animals. The effect of this operation on the weight, contractile properties and force of the extensor digitorum longus muscle was examined at two different ages, i.e. at 2–3 months (young group) and at 5–8 months (old group). The changes with age in both the control and overloaded muscles of normal and mdx mice are also described. The values obtained from the overloaded muscles were always compared with those for the control, unoperated extensor digitorum longus.
2. In the normal strain of mice the weight of the overloaded extensor digitorum longus muscle in the younger group was increased and it remained higher in the older animals. In the mdx mice the overloaded extensor digitorum longus muscles weighed more in the younger animals but not in the older group of mice.
3. The twitch and tetanic tensions of the overloaded muscles were slightly, but not significantly, increased in the younger group of mdx mice, whereas in the older animals there was a significant decrease in both twitch and tetanic tensions.
4. Thus the overloaded muscles from mdx mice progressively deteriorated with age. In both strains of mice the overloaded muscles become less fatiguable with time.