1. The action of pancuronium on transmembrane sodium conductance was investigated in dorsal root ganglion neurones of chick embryos. The Na+ current was measured by use of the patch-clamp technique in whole-cell configuration. 2. Externally perfused pancuronium (50 microM to 1 mM) reversibly inhibited the current by a fast mechanism of action. Inhibition was concentration-dependent (with a half-effective dose of 170 microM) but not voltage-dependent. 3. The activation and inactivation kinetics of the Na+ current were estimated in pancuronium and in control solution by fitting experimental data with a Hodgkin-Huxley theoretical model. 4. The activation time constant tau m, at negative membrane voltages, was larger in the presence of pancuronium than in the control. In contrast, the inactivation time constant tau h was smaller during drug perfusion at membrane voltages < -10 mV. The steady-state inactivation h infinity was not affected by pancuronium. 5. These results suggest that pancuronium may reduce the sodium current by interacting with the sodium channels in both the resting and open states.