1. Hyperexcitability in denervated skeletal muscle is associated with the expression of SK3, a small-conductance Ca2+-activated K+ channel (SK channel). SK currents were examined in dissociated fibres from flexor digitorum brevis (FDB) muscle using the whole-cell patch clamp configuration. 2. Depolarization activated a K+-selective, apamin-sensitive and iberiotoxin-insensitive current, detected as a tail current upon repolarization, in fibres from denervated but not innervated muscle. Dialysis of the fibres with 20 mM EGTA in the patch pipette solution eliminated the tail current, consistent with this current reflecting Ca2+-activated SK channels expressed only in denervated muscle. 3. Activation of SK tail currents depended on the duration of the depolarizing pulse, consistent with a rise in intracellular Ca2+ due to release from the sarcoplasmic reticulum (SR) and influx through voltage-gated Ca2+ channels. 4. The envelope of SK tail currents was diminished by 10 microM ryanodine for all pulse durations, whereas 2 mM cobalt reduced the SK tail current for pulses greater than 80 ms, demonstrating that Ca2+ release from the SR during short pulses primarily activated SK channels. 5. In current clamp mode with the resting membrane potential set at -70 mV, denervation decreased the action potential threshold by approximately 8 mV. Application of apamin increased the action potential threshold in denervated fibres to that measured in innervated fibres, suggesting that SK channel activity modulates the apparent action potential threshold. 6. These results are consistent with a model in which SK channel activity in the T-tubules of denervated skeletal muscle causes a local increase in K+ concentration that results in hyperexcitability.