The experiments involve study of axon reaction in the red nucleus of the cat induced by unilateral interruption of the rubrospinal tract at the C-2 level. In adult cats histological and qualitative histochemical techniques allow determination of the number of cells in the nucleus ruber undergoing chromatolysis as a result of the operative procedure and give some information on changes in numbers and disposition of organelles, such as lysosomes, which can be identified in histochemical preparations. Also determination of changes in cell volume and numbers can be made. Because the rubrospinal fibers are entirely crossed, axon reaction is limited to the nucleus contralateral to tractotomy. Histoautoradiography, electron microscopy and methods for determination of prograde axoplasmic flow (fast and low components) are also applicable to investigation of the nature of axon reaction in these intrinsic CNS neurons.