The central program for interaction between the hind limbs, expressed as the time structure of motor discharges in the nerves to the various muscles, was studied in immobilized decerebrate spinal cats during fictitious locomotion. The program of hind limb interaction (alternating or inphase) in the decerebrate cats was shown to be determined by the relations between the flexor hemicenters. The activity of the latter is either antiphased or cophased. The character of activity of the extensor hemicenters is determined secondarily on account of alternating interaction of each of them with the ipsilateral flexor hemicenter. After injection of dopa into the animals the cophased program of hind limb interaction may be determined by the cophased working of the extensor center.