Detailed investigation of mammalian myelinated nerve fibers began only a couple of years ago. The initial findings were surprising in that they indicated an absence of potassium channels. Moreover, reports indicating that there may only be extranodal potassium channels in these preparations have appeared. These apparent findings upset our ability to make use of the vast knowledge accumulated thus far from other preparations, for the understanding of the excitability mechanisms of mammalian nerves, which are perhaps the most significant for human physiology and pathophysiology. In a preliminary work we demonstrated, using the Nonner type voltage clamp, the presence of potassium channels in normal rat fibers. These were similar in many respects to those of the frog and squid and different in others. In view of the above, the object of this work is to study in detail the ionic conductances in rat sensory and motor fibers and especially their potassium conductance system. Both the nodal and extranodal channels described by Chiu et al. (1980), if they are separate entities, will be studied. On the basis of the results we will try to gain insight into the potassium channel gating kinetics and basic membrane mechanisms, construct a mammalian axon model, reconstruct the mammalian myelinated nerve excitability properties and compare them with the experimental results.