An abnormal ratio between Na⁺ and K⁺ conductances seems to be the cause for the depolarization and paralysis of skeletal muscle in primary hypokalemic periodic paralysis. Recently we have shown that the „k⁺ channel opener”︁ cromakalim hyperpolarizes mammalian skeletal muscle fibers. Now we have studied the effects of this drug on the twitch force of muscle biopsies from normal and diseased human skeletal muscle. Cromakalim had little effect on the twitch force of normal muscle whereas it strongly improved the contraction force of fibers from patients suffering from hypokalemic periodic paralysis. Recordings of intracellular K⁺ and Cl⁻ activities in human muscle and isolated rat soleus muscle support the view that cromakalim enhances the membrane K⁺ conductance (gK⁺). These data indicate that ”︁K⁺ channel openers„ may have a beneficial effect in primary hypokalemic periodic paralysis.