The same protocol was used three times to produce a paralytic episode in a typical case of periodic familial paralysis with hypokalemia. This consisted of an effort together with a perfusion of hypertonic glucose serum and insulin. The first test provoked an attach of hypokalemic tetraplegia. The second test, two months after treatment with 500 mg daily of acetazolamide, produced no reaction. In the third test, the metabolic acidosis caused by acetazolamide was reduced by the injection of sodium bicarbonate, and a stronger effect than in the first test was observed. This confirms the efficacy of acetazolamide as a preventive treatment for paralytic attacks; the most reasonable hypothesis being that it acts through the metabolic acidosis that it induces. Metabolic and hemodynamic studies were carried out during the provoked attacks. Cardiac output and oxygen consumption are increased, while pulmonary capillary pressure and periopheral resistance are reduced. Diastolic pressure is lower when measured by an arm-cuff but shows no change when direct readings are taken in the blood-vessels. These results suggest that there is an increase in cellular energy needs, or that the smooth muscle in the vessel walls is paralyzed. The two tetraplegic attacks in tests 1 and 3 were associated with a metabolic acidosis, which is explained by a simultaneous transfer, though in the opposite direction, of H+ and K+ ions; the intra-cellular pH, as measured by the D.M.O. technique, was acid when there was not an attack, and this increased during paralysis.