The effects of signals proceeding along descending systems of the spinal cord on the parameters of efferent activity in the scratch generator (i.e., discharges in ventral roots L5 and L7) were investigated in decerebrate immobilized cats. It was found that eliminating cerebellar modulatory influences on nuclei of the descending system (decerebellation) leads to an altered ratio between the activity of generator hemicenters, increased instability of generator operation, a reduced statistical relationship between alteration in parameters of hemicenter activity, and raised sensitivity to afferent signals in the generator. Complete elimination of descending system influences (after severing the spinal cord at the anterior boundary of the first cervical segment) produced a reduction in the intensity of efferent activity and in the afferent sensitivity of the scratch generator. The functional significance of descending signals for operation of the scratch generator is discussed.