16 patients suffering from a hyperventilation syndrome were treated with metoprolol 2dd 100 mg and an identical placebo in a double-blind, cross-over trial. Before therapy and after metoprolol and placebo therapy a ventilatory response to CO2 was taken, VC and FEV1 a hyperventilation provocation test, blood gas values, and the subjective experiences of the patients were documented. The ventilatory response to CO2 was described in terms of decrease or increase of ventilation: before therapy ventilation decreased in 10 out of 16 patients, after metoprolol ventilation decreased in 3 of 16 patients (p < 0.01). The end tidal Pco2 increased with a mean of 3.86 mm Hg (p = 0.0005) after metoprolol as compared to placebo. No differences were found in respiratory frequency or depth, base excess, provocation test. It is concluded that the cardioselective beta-blocker metoprolol is a useful drug in the therapy of the hyperventilation syndrome