The cause of weakness was investigated in a patient with adynamia episodica hereditaria without myotonia. A pattern of exercise and rest produced episodes of hyperkalemic periodic paralysis. In addition, local muscle weakness was induced by forearm cooling. Investigations on isolated intercostal muscle demonstrated that a high potassium concentration in the bathing solution triggered a noninactivating membrane current causing depolarization of the muscle fibers. This current was carried by sodium as it could be inhibited by tetrodotoxin. The abnormal sodium conductance led to an increase of sodium within the fibers. This was demonstrated directly by intracellular recordings. Weakness induced by rest after exercise and cold‐induced weakness appeared to have different pathomechanisms. In the cold, the muscle fibers retained a normal resting potential, but their excitability was reduced and their mechanical threshold was increased. These findings also provide evidence that the mechanism of cold‐induced weakness in adynamia episodica is distinctly different from the cold‐induced weakness that occurs in paramyotonia congenita.