In the present work, we have investigated the effects of the aging process on Ca²⁺-activated K⁺ channels (K_Ca2+) of rat skeletal muscle fibres. K_Ca2+ channels of adult (5–7 months old) and aged (24–26 months old) rats were surveyed by the patch-clamp technique. In aged rats, K_Ca2+ channels were routinely detected on the surface membrane of the fibres in both cell-attached and inside-out configurations. Conversely, in adult rat fibres, K_Ca2+ channels were rarely detected. In the cell-attached configuration, the open probability of the aged rat K_Ca2+ channel, measured in the range of potentials from –60 mV to +20 mV, was about 1.5–2 times higher than that of the adult one. The number of functional channels was abnormally increased by aging. An average of three channels per patch/area was counted in the inside-out patches of aged rat fibres, whereas no more than one open channel per patch/area was detected in the adult rat fibres. The frequency of finding channels in the patches also increased with aging, i.e. 11.5% and 30.1% in the adult and in the aged rat fibres, respectively. However, no significant change in the single-channel conductance has been observed with aging: it was 227 pS and 231 pS for adult and aged rat channels, respectively. In detached patches, both the adult and aged rat channels showed a similar voltage dependence of open probability and a similar sensitivity to Ca²⁺ ions. The aging process did not alter the response of the single channel to charybdotoxin, or its modulation by nucleotides, MgATP and adenosine 5’-O-(3-thiotriphosphate) (ATP[γ-S]). On the other hand, charybdotoxin reduced the abnormally high resting macroscopic K⁺ conductance of the aged rat fibres, recorded using the two-intracellular-microelectrode technique. These findings indicate that, in skeletal muscle, the activity of K_Ca2+ channels increases with advancing age.