Under electrophysiological control, a focal demyelinating lesion can be produced in the corpus callosum of the rabbit by slow pressure injection of a 1% solution of lysophosphatidyl choline (LPC). Light- and electron-microscopic examination of the LPC demyelinated corpus callosum indicates that many axons remain structurally intact after LPC injection. In addition, some axons show early signs of remyelination. Our results indicate that small diameter, central nervous system myelinated axons can be focally demyelinated with LPC. Furthermore, our procedure for producing demyelination in the corpus callosum is particularly suitable for combined anatomical and electrophysiological study of small demyelinated axons.