Hyperkalemic periodic paralysis is frequently considered a disorder in which episodes of weakness and an attendant rise in plasma potassium interrupt a baseline of normal strength and potassium. We studied venous potassium throughout a 36-hour period in two patients with hyperkalemic periodic paralysis and in nine normals under rigidly controlled conditions. At no time did the patients with periodic paralysis have an attack of weakness, but their mean potassium concentrations were above the normal range for 33 to 36 hours. In hyperkalemic periodic paralysis, the postprandial change in potassium relative to insulin release exceeded normal. There appears to be a continuous alteration in potassium regulation in our patients with hyperkalemic periodic paralysis.