1. Using cell-attached patch pipettes, sustained activity of single potassium M-channels was recorded from dissociated rat superior cervical ganglion neurones. Previous results indicated that this activity, consisting of three main levels of open-channel conductance (congruent to 7, congruent to 12 and congruent to 19 pS) was activated by membrane depolarization and inhibited by muscarine added outside the patch. Consequently, a kinetic analysis was undertaken in order to identify M-channel states sensitive to muscarine and membrane potential. 2. Channel activity recorded at 30 mV positive to the resting membrane potential level (congruent to -60 mV) showed three shut and two open times. Mean shut times were: tau s1 = 8.0 +/- 2.2 ms; tau s2 = 71.3 +/- 8.6 ms and tau s3 = 740 +/- 220 ms. Mean open times were: tau o1 = 10.6 +/- 1.9 ms and tau o2 = 59.3 +/- 8.7 ms. When bursts of channel openings were determined as those including tau s1, two exponential components were evident in burst duration distributions (tau b1 = 11.0 +/- 0.9 ms and tau b2 = 80.4 +/- 11.0 ms). 3. Membrane hyperpolarization significantly lengthened all three shut times and shortened both open times. It also slightly enhanced the relative contribution of high-conductance channels and decreased the relative contribution of low-conductance channels to overall activity. 4. All three shut times of the M-channels were lengthened by 10 microM muscarine without significantly affecting their open times. 5. It is concluded that both open and shut states of the M-channel are voltage sensitive while only shut states are sensitive to muscarine.