Quantitative electron microscopic examination was made of Betz cells of two unoperated cats as well as cats subjected to left lateral funiculotomy 5, 10, 28 and 49 days before sacrifice. The percent cytoplasmic composition of chromatolyzed, right-sided Betz cells contributed by cisternal elements of RER, Golgi apparatus and dense bodies and the percent perikaryal membrane apposed by subsurface cisterns were unchanged from the normal despite marked qualitative alterations of the cytoplasm. However, 49 days postoperatively mitochondrial numerical density of axotomized, right-sided Betz cells was significantly less than at 0, 10 and 28 days post funiculotomy. Importantly, normal-appearing Betz cells ipsilateral to corticospinal tract section showed an increase in mitochondrial numerical density 5 days postoperatively. Operation did not induce change in the % perikaryal coverage by axosomatic boutons. Retraction of axosomatic boutons, though often reported for other neuronal populations undergoing axon reaction, is not a necessary feature of the axon reaction of feline Betz cells.