1. Whole-cell recordings of M-currents and single-channel recordings have been made in cultured rat sympathetic ganglion (SCG) neurones using the patch clamp technique. 2. Muscarine caused a reduction in macroscopic M-current relaxations, induced by voltage steps, and a concomitant reduction in whole-cell current noise. Power spectra of the muscarine-sensitive component of current noise were fitted with two Lorentzian components corresponding, on average, to 162 and 15 ms. The longer time constant was very similar to that of deactivation tail currents measured at the same potential. 3. The single-channel conductance at -30 mV was estimated from power density spectra and whole-cell current-variance relationships to be 1-2 pS. 4. Putative single M-channels, activated by depolarization, were identified in cell-attached and outside-out patches from cultured SCG neurones. In particular, the ensemble average of a small amplitude channel (estimated to be ca4 pS in physiological [K+]) in a cell-attached patch, exhibited a similar time dependence to whole-cell M-current.