The ion conduction and voltage dependence of sodium channels purified from rat brain were investigated in planar lipid bilayers in the presence of batrachotoxin. Single channel currents are clearly resolved. Channel opening is voltage dependent and favored by depolarization. The voltage at which the channel is open 50% of the time is -91 +/- 17 mV (SD, n = 22) and the apparent gating charge is approximately 4. Tetrodotoxin reversibly blocks the ionic current through the sodium channels. The Ki for the tetrodotoxin block is 8.3 nM at -50 mV and is voltage dependent with the Ki increasing e-fold for depolarizations of 43 mV. The single channel conductance, gamma, is ohmic. At 0.5 M salt concentrations gamma = 25 pS for Na+, 3.5 pS for K+, and 1.2 pS for Rb+. This study demonstrates that the purified brain sodium channel--which consists of three polypeptide subunits: alpha (Mr approximately 260,000), beta 1 (Mr approximately 39,000), and beta 2 (Mr approximately 37,000)--exhibits the same voltage dependence, neurotoxin sensitivity, and ionic selectivity associated with native sodium channels.