1. Acute hypocapnic hypoxia was induced in ten healthy subjects by the inhalation of O2+N2 (10:90) at rest and during exercise. Transient relief of hypoxia by two breaths of O2 (O2 test) resulted in a decrease in cardiac frequency which was highly significant about 10 s after the tests and never consistently delayed in comparison with the ventilatory changes.

2. β-Adrenergic blockade with propranolol reduced the magnitude of the changes in cardiac frequency but not their time-course.

3. Cholinergic blockade with atropine sulphate (and double blockade) abolished the early changes in cardiac frequency. During continued O2 breathing (3 min) small, still significant, changes were observed.

4. It is concluded that the decrease in cardiac frequency within 10 s of relief of hypoxia is dependent on efferent vagal activity.

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