ALTHOUGH it has long been known that a striking fall in serum potassium level may coincide with the attacks of weakness occurring in familial periodic paralysis, and that the administration of potassium salts may relieve such an attack,^{1 2 3} other families suffering from similar attacks have been described in which no such change in potassium level or relief followed potassium administration.^{4 , 5} In 1956 Gamstorp⁶ described in detail 17 cases from two families suffering from periodic weakness with normal or slightly elevated serum potassium levels during attacks and a striking response to orally administered potassium chloride, which, instead of relieving an attack, . . .