In chronic spinal cats walking with the hind legs on a treadmill, one limb can be stopped by holding the paw while the other limb continues to walk. If the held limb is slowly brought backwards at one point the limb flexes and continues walking. It was found that the hip position at which the leg lifts off during such reaction is very close to the hip angle at the initiation of swing during locomotion. Similar findings were obtained by extending only the femur. The hand-held limb tends to initiate lift-off during the midstance or the midswing of the contralateral limb step cycle. It is concluded that hip position and the contralateral step cycle phase are two important factors determining the initiation of swing in one leg.