Controlled ventilation, particularly with mechanical ventilators, invariably leads to some degree of hyperventilation. The biochemical alterations produced by hyperventilation are harmless in the healthy patient, and are easily reversible. The main electrolytic change is a reduction serum potassium. This may be of significance in the digitalized patient, where it may produce toxicity. Hyperventilation, combined with other methods, is a satisfactory technique for reducing brain volume in selected intracranial operations. Mechanical ventilation of the ill, post-operative patient is often a life-saving procedure and is almost mandatory following open heart surgery. Hyperventilation of patients with chronic obstructive airway disease improves their alveolar ventilation and produces marked clinical improvement. Increased irritability of the central nervous system may occur, leading to convulsions.