The soleus (SOL) or extensor digitorum longus (EDL) muscles of month-old rats were denervated for 14 days and then cross-transplanted so that the fast muscle was placed into the bed of the slow muscle and vice versa. At 17, 30, 60, and 90 days the transplants were tested for certain contractile and histochemical properties. By 90 days the cross-transplanted SOL showed complete conversion of the full contraction time and nearly complete conversion of the half relaxation time to those of the normal EDL. In contrast, the contraction and relaxation times of the cross-transplanted EDL became considerably slowed, but did not attain the values of the normal SOL. Histochemical staining for ATPase and SDH activity demonstrated similar transformations of fiber types. The degree of transformation of twitch and histochemical characteristics in cross-transplanted muscles was greater than the values reported after cross-innervation of the same muscles. The cross-transplantation model has certain advantages over nerve cross-union experiments because the cross-transplanted muscle is placed in the normal functional environment of the other muscle.