Properties of acetylcholine(ACh)-and nicotine-induced currents were studied in the guinea-pig chromaffin cell, using the whole-cell and cell-attached versions of the patch-clamp technique. Bath application of ACh or nicotine, but not muscarine, produced an inward current associated with an increase in current noise at a membrane potential of -70 inV. Low concentrations of both agonists produced a sustained inward current whereas high concentrations produced a transient, then a sustained inward current. Nicotine was about twice as potent as ACh in inducing the inward current. Hexamethonium (6 ~tM) inhibited the ACh-induced current but not in a competitive manner. By contrast, atropine (6 ~tM) inhibited the ACh-induced current more strongly with increasing concentrations of ACh. The nicotinic-receptor-activated current ("nicotinic current") showed inward rectification and, when Cs Β§ was used instead of K + in the pipette solution, the polarity of the current changed at around -5 mV and a negative slope occurred between + 10 mV and + 30 InV. The nicotinic channel had a unit conductance of 33pS. During the initial 20-30 min of whole-cell voltage-clamp recording, the time course of the agonist-induced desensitization was markedly facilitated. Addition of 5 mM MgATP in the pipette solution at least partly prevented this facilitation of the desensitization. The frequency of activation of the nicotinic receptor and the extracellular Ca 2 Β§ were not primary factors in the acceleration of rate of desensitization.