The actions of halothane on single-channel currents evoked by acetylcholine in myoballs cultured from neonatal rat skeletal muscle have been investigated using the patch clamp technique. Halothane at concentrations of 0.5-4.0 mM was applied to inside-out cell-free membrane patches in the presence of 1.0 microM-acetylcholine. Halothane produced concentration-dependent decrease in the mean duration of the single-channel current events; 3.0 mM-halothane halved the slow time constant of channel closure. There was no effect on the amplitude of the single-channel currents, and no increase in the number of closing events per burst was observed. The distribution of closed times was complex but halothane did not markedly alter the frequency of channel opening events. Halothane therefore decreased the time-averaged acetylcholine-evoked membrane current, largely through an increase in the rate of channel closure.