The action potentials of some normal mammalian peripheral myelinated and non-myelinated nerve fibers are known to differ with respect to the role of voltage-dependent potassium currents in membrane repolarization. Because comparable differences have not been fully established for mammalian cerebral axons, the present work examined the influence of the potassium blocker, 4-aminopyridine (4-AP), on field potentials in the rat corpus callosum. Three successive negative waves could be detected in field potentials obtained with recording electrodes positioned several millimeters from the site of callosal stimulation. Spatial and functional properties of these field components indicated that the two shorter-latency waves were due to activity in callosal fibers. Furthermore, the discontinuity of the first and second waves, not only with respect to latency but also in pattern of recruitment and in refractoriness, suggested that they largely reflected activity in respective myelinated and nonmyelinated groups of callosal fibers. When 4-AP was injected into the callosum, or superfused onto surgically exposed callosal fibers, the duration and amplitude of the second negative wave were markedly increased. The first negative wave either was not affected or was reduced in amplitude by 4-AP application. This contrasting effect was identical to that known to distinguish normal myelinated and nonmyelinated mammalian peripheral nerve fibers and provided some evidence suggesting that mammalian cerebral axons have a functional organization similar to that of peripheral fibers.