Dorsiflexion of one hind foot has been prevented in guinea pigs by means of a light prosthesis. The gastrocnemius and soleus muscles of both hind legs have been removed after a period of 2 weeks, and the fiber cross-sectional areas of type 1 and 2 fibers in both muscles have been measured. Comparison of the values in normal and affected muscles shows that the limited reduction in free movement of the foot led to a significant atrophy in types 1 and 2 fibers in both muscles. The use of this simple prosthesis appears to have no effect on any other function of the leg except for the restriction of the lengthening of gastrocnemius and soleus muscles, and the results show that some of the hypotheses about the muscle atrophy seen in fully immobilized limbs cannot account for the changes observed in the present experiments and seem to be of limited validity.