The symptom patterns of the hyperventilation syndrome have been recognized for many years.1-5Some workers have reported abnormal lowering or inversion of the T-wave and, less frequently, depression of the ST segment in the routine electrocardiogram of many patients with this syndrome or with anxiety and emotional instability.6-13These changes are readily reproduced by voluntary hyperventilation. Several others have observed similar changes of the T-wave in the electrocardiogram of certain normal persons during voluntary hyperventilation.14,15 The purpose of this report is (1) to describe the electrocardiographic abnormalities during voluntary hyperventilation in 20 patients with the hyperventilation syndrome and the effects of inhalation of CO2and administration of various drugs in these changes; (2) to present the changes of serum electrolytes, respiratory gases, and pH of the blood in these patients during hyperventilation, with or without inhalation of CO2; (3) to postulate a possible mechanism for the electrocardiographic changes, and (4)