Single-channel currents underlying the various types of nicotinic receptor-gated whole-cell currents (previously termed IA, IB, II and III) were identified in rat hippocampal neurons. In response to applied acetylcholine (ACh), most of the neurons showed a fast-decaying whole-cell current (type IA) that can be blocked by alpha-bungarotoxin (alpha-BGT). In these neurons, a novel nicotinic receptor channel was found, having a conductance of 73 pS and an open time of 0.12 ms at -80 mV. This channel showed a fast concentration-dependent inactivation that had a time constant of 0.5 ms at 1 mM ACh. A high Ca2+ permeability and the involvement of alpha 7 receptor subunits in the channel structure were suggested.