Abstract. To detect cation channels, the expression of which is dependent on the physiological state of muscle, single-channel activities of dissociated fibres of the mouse interosseus muscle were recorded using the patch-clamp technique in the cell-attached mode. Fibres were prepared from juvenile and adult wild-type (WT), from chloride channel-deficient myotonic and from denervated adult WT muscles. In all cases delayed-rectifier K+ channels (KDR) with a unitary conductance of 11pS were recorded in more than 95% of sarcolemmal patches, but with a low, steady-state open probability. Inwards-rectifying K+ channels (KIR) with a conductance of 31pS in 140mM [K+]o were active in about 50% of the membrane patches from WT and in more than 90% of those from myotonic fibres. A hitherto undescribed, inwards-rectifying, cation channel, provisionally termed CIR, with fast kinetics and a unitary conductance of 36pS, was active in nearly every membrane patch from juvenile mice, both WT and myotonic. The abundance of CIR decreased during development, but was not changed 7days after denervation of adult WT muscle. Ca2+-dependent K+ channels were seen sporadically. Channels with the characteristics of adenosine 5'-triphosphate (ATP)-sensitive K+ channels were recorded frequently upon excision of membrane patches, but remained inactive in most cell-attached recordings. In conclusion, of the investigated ion channels, only KIR was responsive to the activity pattern of adult muscle, whereas CIR was down-regulated during muscle maturation.