- 1. In whole cell recordings from layer V neurons in slices of mouse somatosensory neocortex, tetrodotoxin (TTX)-sensitive persistent Na+ current (INaP) was studied by blocking K+ currents with intracellular Cs+ and Ca2+ currents with extracellular Cd2+. During slow voltage ramps, INaP began to activate at around -60 mV, and attained a peak at around -25 mV. The peak amplitude of INaP varied widely from cell to cell (range 60-3,160 pA; median 308 pA, n = 77). At potentials more positive than -35 mV, INaP in all cells was superimposed on a large, TTX-resistant outward current. 2. In hybrid clamp experiments, INaP was significantly reduced by a preceding high-frequency train of spikes. 3. INaP underwent pronounced slow inactivation, which was revealed by systematically varying the ramp speed between 233 and 2.33 mV/s, or varying the duration of a depolarizing prepulse between 0.1 and 10 s. 4. Onset of slow inactivation at +20 mV was monoexponential with tau = 2.06 s (n = 17 cells). Recovery from slow inactivation was voltage dependent. It followed a monoexponential time course with tau = 2.31 s (n = 6) at -70 mV and tau = 1.10 s (n = 4) at -90 mV. These values are not significantly different than values previously reported for slow inactivation of fast-inactivating INa. 5. Slow inactivation of neocortical INaP will influence all neuronal functions in which this current plays a role, including spike threshold determination, synaptic integration, and active propagation in dendrites. The kinetics of slow inactivation suggest that it may be a factor not only during extremely intense spiking, but also during periods of "spontaneous" activity.