A fourth variant of familial periodic paralysis (FPP) is described. The syndrome is characterized by premature ectopic heartbeats of varying locus and frequency, but most commonly manifested by bigeminy. The paralytic attacks are otherwise indistinguishable from those which occur in other forms of FPP. Serum potassium values are occasionally elevated but this is only coincidentally related to paralytic attacks. The disease is aggravated by increased sodium intake, digitalis, cold weather, and is more frequent premenstrually. Increased potassium intake, glucose intake, and a low-sodium diet or spironolactone are both therapeutic and prophylactic. Quinidine is an effective therapeutic agent. Hypotheses have been advanced to explain the mechanism of paralysis in several forms of FPP.