Properties of the persistent sodium conductance and the calcium conductance of layer V neurons from cat sensorimotor cortex were examined in an in vitro slice preparation by use of a single microelectrode, somatic voltage clamp, current clamp, intra- and extracellular application of blocking agents, and extracellular ion substitution. The persistent sodium current (INaP) attained its steady level within 2-4 ms of a step change in voltage at every potential where it could be examined directly [to about 40 mV positive to resting potential (RP)]. Because of its fast onset INaP can be activated during a single excitatory postsynaptic potential (EPSP) and can influence the subsequent voltage time course and cell excitability. Application of a depolarizing holding potential greater than or equal to 20 mV positive to RP could inactivate spikes, thus allowing examination of INaP at voltages positive to spike threshold. At every potential where INaP was visible, it was mixed with a slow outward current. After depressing potassium currents with blocking agents, INaP could be observed during depolarizations to about 40 mV positive to RP where it is normally hidden by the larger outward currents. Indirect evidence suggests that INaP is present and large during prolonged depolarizations greater than 50 mV positive to RP. INaP was blocked by intracellular injection of the lidocaine derivative QX-314, as well as by extracellular tetrodotoxin (TTX). INaP was much more sensitive to QX-314 than was the height and rate of rise of the spike. This observation and the results in paragraph 3 above are best explained by separate INaP and spike sodium channels. After blockade of INaP and sodium spikes, Ca2+ spikes could be evoked only if potassium currents were first depressed. The Ca2+-dependent nature of the regenerative potentials was indicated by their disappearance when Co2+ or Mn2+ was substituted for Ca2+ in the perfusate and by the appearance of greatly enhanced potentials of similar form when Ba2+ was substituted for Ca2+. Ba2+ substitution greatly enhanced evoked and spontaneous synaptic potentials. Prolonged-plateau action potentials could be evoked in the presence of TTX and Ba2+. Ca2+ spike threshold was 30-40 mV positive to RP, which is significantly more positive than sodium spike threshold. Results of voltage clamp in the normal perfusate and in the presence of Ca2+-blockers or Ba2+ indicated that little or no Ca2+ conductance is activated in the voltage range 25 mV positive to RP where INaP is the dominant ionic current.(ABSTRACT TRUNCATED AT 400 WORDS)