In the body fluids of patients suffering from acute mercury poisoning the changes in the concentration of electrolytes may be profound. Few other diseases are associated with a greater disturbance of acidbase equilibrium than is observed in this condition. Addison's disease,1 diabetic coma2 and heat cramps3 are examples of morbid conditions which are associated with recovery as the disordered equilibrium is corrected. Advanced renal insufficiency, on the other hand, is a disease with electrolyte imbalance which resists restoration to normal and ends with a disintegration of the chemical structure of the body.4 Acute mercury poisoning sometimes belongs in one category, sometimes in the other. In this communication there will be described the several changes in the concentration of electrolytes in a patient who recovered from acute and severe mercury poisoning. The amount of mercury bichloride which was ingested was believed to have been 1 Gm. Usually