The effects of hind limb suspension on the development of dystrophic muscles was studied in five dystrophic hamsters (CHF 147, formally UMX 7.1) from age 20 days for 5 months. Their histochemical and contractile properties were compared to five aged-matched controls. Twitch and tetanic tensions (Po) were reduced in treated soleus by 22% and 32% and in plantaris by 29% and 39%, respectively; these reductions were proportional to their smaller cross-sectional areas. Twitch duration and half-relaxation times were not altered in experimental soleus muscle but the mean time to 50% of Po was 19 ms faster than the controls. Experimental soleus and plantaris had higher percentages of type II fibers, i.e., 63% and 97% compared with 37% and 93% in controls, respectively. A similar trend was seen in gastrocnemius and extensor digitorum longus muscles. Areas of both fiber types were reduced in all muscles, but only statistically significant differences in type I areas were seen in soleus and type II areas in the other three muscles. The percentages of fibers with centronuclei in whole cross sections, determined at different positions along the muscle lengths, were reduced by 30% in soleus and extensor digitorum longus. The areas of fiber necrosis were also reduced in experimental extensor digitorum longus and plantaris. These changes in nonweight-bearing muscles support the hypothesis that contractile activity can influence the development of dystrophic properties.