In a 19 yr old male with familial hyperkalemic periodic paralysis, paralysis was consistently induced by the administration of potassium chloride, corticotropin-gel, and a variety of glucocorticoids (dexamethasone, 6-methylprednisolone, triamcinolone) but not by mineralocorticoids (D-aldosterone, deoxycorticosterone) or by adrenocorticotropin (ACTH)-gel plus metyrapone. Induced attacks were virtually identical with spontaneous attacks, being associated, after a latent period of a few hours, with a rise in plasma K(+) and HCO(3) (-) and a simultaneous fall in plasma Na(+) and Cl(-) concentrations to an extent implying exchange of 1 K(+) with 2 Na(+) and 2 Cl(-) between extracellular and intracellular fluid. ACTH-induced paralysis was preceded by rising serum inorganic P, and associated with increased plasma glucose, blood lactate, and serum creatine phosphokinase concentrations. In normal subjects ACTH, cortisol, and triamcinolone administration failed to change plasma electrolytes or strength, while ingestion of KCl produced no weakness and smaller changes in plasma K and Na than in the patient.Since the patient and normal subjects showed the same changes in renal excretion of K after the administration of cortisol and KCl, it seems likely that paralysis in the patient resulted from abnormally slow uptake (and/or excessive loss) of K by the muscle cells, possibly caused by an abnormal "ion-exchange pump." Normal adrenocortical function and absence of a peak in plasma 11-hydroxycorticoid (11-OHCS) concentration preceding spontaneous paralysis, indicated that spontaneous paralysis did not result from changes in cortisol secretion. Similar hyperkalemic paralysis was precipitated by ACTH-gel in a brother and first cousin of the propositus. Administration of acetazolamide and fludrocortisone reduced the rise in plasma K concentration and prevented the weakness which otherwise invariably followed KCl administration to the patient. He and two close relatives have been completely protected from severe attacks of paralysis in the past 14 months by treatment with these two medications.