The functional and biophysical properties of a persistent sodium current (I NaP) previously proposed to participate in the generation of subthreshold oscillations and burst discharge in mesencephalic trigeminal sensory neurons (Mes V) were investigated in brain stem slices (rats, p7-p12) using whole cell patch-clamp methods. I NaP activated around -76 mV and peaked at -48 mV, with V 1/2 of -58.7 mV. Ramp voltage-clamp protocols showed that I NaP undergoes time- as well as voltage-dependent inactivation and recovery from inactivation in the range of several seconds (τ onset = 2.04 s, τ recov = 2.21 s). Riluzole (≤5 μM) substantially reduced I NaP, membrane resonance, postinhibitory rebound (PIR), and subthreshold oscillations, and completely blocked bursting, but produced modest effects on the fast transient Na + current (I NaT). Before complete cessation, burst cycle duration was increased substantially, while modest and inconsistent changes in burst duration were observed. The properties of the I NaT were obtained and revealed that the amplitude and voltage dependence of the resulting "window current" were not consistent with those of the observed I NaP recorded in the same neurons. This suggests an additional mechanism for the origin of I NaP. A neuronal model was constructed using Hodgkin-Huxley parameters obtained experimentally for Na + and K + currents that simulated the experimentally observed membrane resonance, subthreshold oscillations, bursting, and PIR. Alterations in the model g NaP parameters indicate that I NaP is critical for control of subthreshold and suprathreshold Mes V neuron membrane excitability and burst generation.