Our knowledge of voltage dependent membrane properties of neurons is fimrly based on the extensive experiments on axons and invertebrate cell bodies. Voltage clamp studies have recently revealed many characteristics of the ionic conductance channels in spinal motoneurons from the intact cat. These studies have shown the presence of two inward currents carried by sodium and calcium and three independent outward currents carried by potassium. Quantitative description of these currents has been hampered by the technical difficulties of impaling motoneurons in the intact cat with two electrodes and the inability to alter extracellular ion concentrations in a precise manner. In this proposal, we plan to extend our voltage clamp studies to the large neurons of the cat neocortex using the cortical slice technique. We will examine these neurons for the presence of various ionic conductance elements similar to those already described in cat motoneurons. Experiments are decribed that will allow us to test for the hypothesized currents in isolation. These studies will be the first detailed examinations of the ionic properties of mammalian neocortical cells. Furthermore we will be able to compare the findings directly with those of spinal motoneurons.