Sarcolemmal vesicles were produced from adult mouse extensor digitorum longus muscle (EDL) by treating swollen muscle fibres with collagenase. Vesicles formed from dystrophic (C57BL/6J dy/dy) and phenotypically normal animals were patch clamped and the single channel activity was recorded. Three types of K+ channel were observed in excised patches taken from normal and dystrophic muscle. A large conductance (300 pS) Ca2(+)-dependent K+ channel (KCa) was the most frequently observed of the K+ channels in both types of muscle preparation. In a number of patches taken from dystrophic muscle the open probability-voltage relationship for the KCa channel was markedly different from that in normal muscle, suggesting a possible reduction in Ca2+ sensitivity. An ATP-sensitive K+ channel (90 pS) was common to both normal and dystrophic muscle vesicles and was present in a large number of patches. An inwardly rectifying K+ channel (40 pS) was also observed in both types of sarcolemmal vesicles. The properties of all three K+ channels types were broadly consistent with other observations of skeletal muscle K+ channels, though all had higher conductances than had previously been noted in other species.