An understanding of the chemistry of the microtubule and of its in vivo response to colchicine administration has been facilitated by the recent work of Taylor and his coworkers (1, 2). Robbins and Gonatas (3) have shown that colchicine causes the breakdown of mitotic spindle tubules of HeLa cells and a simultaneous increase in the apparent number of 90-A microfilaments in the cytoplasm. Similar changes have been observed by Padawer in the mast cell.'Finally, the homology between the spindle tubule and the neurotubule has been suggested (4). After subarachnoid injection of colchicine, there is an accumulation of filaments in the neurons of the spinal cord anterior horn (5). Similar proliferation of neurofilaments is seen after subarachnoid administration of the mitotic spindle inhibitors vinblastine and podophyllotoxin. The experiments reported here show the reversibility