Myotonic muscular dystrophy is a genetic disease characterized mainly by muscle atrophy and myotonia, a repetitive electrical activity of muscle. In the present study, the possible role of apamin‐sensitive K⁺ channels in the genesis of myotonia was investigated. Apamin is a peptide from bee venom that specifically blocks small conductance Ca²⁺‐activated K⁺ channels. The injection of a small amount of apamin (20–30 μl, 10 μmol/L) into the thenar muscle of myotonic dystrophy patients decreased the basal electrical activity during the electromyogram in the 6 patients studied. Myotonic discharges after muscle percussion were more difficult to trigger and of smaller intensity and duration. In 2 controls and in 2 patients with generalized myotonia, as well as in 1 patient with myotonia congenita (where the defect is in chloride channels), apamin had no effect. These results suggest that apamin‐sensitive K⁺ channels participate in the mechanism that generates myotonia in myotonic dystrophy. © 1994 John Wiley & Sons, Inc.