Exercise can abort or postpone attacks of weakness in familial hypokalemic periodic paralysis (FHoPP). To determine its effect during established attacks, muscle contraction was produced by electrical stimulation of the ulnar nerve during attacks of weakness in two sisters with FHoPP. Repeated trains of stimulation produced normal strength and evoked potential amplitude in stimulated muscles without changing the strength or evoked potential of adjacent, nonstimulated muscles. It is suggested that the beneficial influence of muscle activity during attacks of FHoPP is a local membrane event resulting in recruitment of muscle fibers.