1. Properties of the Ca-activated K channel were studied in excised patches of surface membrane from cultured rat muscle cells using single channel recording techniques.2. Increasing the concentration of calcium at the intracellular membrane surface [Ca](i), increased both the frequency and effective duration of channel openings. Ca at the extracellular membrane surface was not sufficient to activate the channels.3. An approximate third power relationship (slope = 2.7) was observed between [Ca](i) and the percentage of time the channels spent in the open state.4. Both the frequency and effective duration of channel openings increased as the intracellular membrane surface was made more positive; the percentage of time spent in the open state increased e-fold for a 15 mV depolarization for low levels of activity.5. The percentage of time spent with 1, 2,...n channels open in membrane patches with n channels was described by the binomial distribution, suggesting that the channels opened and shut independently of one another.6. Single channel conductance (144 mM-K on both sides of the membrane) was essentially independent of membrane potential (-50 to +50 mV) and [Ca](i) (0.1 muM -1 mM), but did increase with temperature, from 100 pS at 1 degrees C to 300 pS at 37 degrees C.7. Channel activity occurred in apparent bursts, with the duration of the apparent bursts increasing with increasing [Ca](i).8. Two exponentials were required to describe the distribution of observed channel open times, suggesting two different open channel states of apparently normal conductance. The observed mean channel open time of these states at +30 mV was 0.34 and 2.2 msec with 0.1 muM-Ca(i) and was 0.47 and 6.9 msec with 0.5 muM-Ca(i).9. The channel occasionally entered an apparent third open channel state with a single channel current amplitude about 40% the amplitude of the normally observed single channel currents. The reduced conductance state was immediately preceded and followed by a normal conducting state.10. While the kinetics of the Ca-activated K channel appear complex, its large conductance and high Ca and voltage sensitivity suggest that it is uniquely suited to resist depolarizations of the cell membrane potential that are accompanied by increases in intracellular Ca.