The effect of β-adrenergic blockade on ventilatory parameters and on subjective complaints was studied in 73 patients suffering from hyperventilation syndrome. The β-blockade made the PA,co2 increase as much as tranquillizer therapy. No effect of any kind of therapy was found on the subjective complaints. A remarkable dissociation was found between the measured ventilatory parameters and the quantified subjective complaints. It is concluded that β-blockade is an effective therapy for the basic problem of hypocapnia in the hyperventilation syndrome; in this respect it is to be preferred over other therapies such as tranquilliz