1. Oxygen breath tests were performed in seven patients with thyrotoxicosis. The inspired air was replaced by oxygen without their knowledge for 20 s and the fall in ventilation was recorded.
2. Ventilation fell significantly in all the patients within 40 s. The mean fall was 21·1% (se ± 1·7) below the control value.
3. β-Adrenoreceptor blockade with propranolol had little effect on this reduction in ventilation. The fall observed in five patients receiving propranolol was 19·6% (se ± 1·0).
4. After definitive treatment, which rendered the patients euthyroid, the observed reduction in ventilation after breathing oxygen was significantly less at 11·7% (se ± 1·2).
5. It is concluded that patients with thyrotoxicosis have an increased resting reflex hypoxic drive to respiration, when compared with the euthyroid state, and this is unaffected by β-adrenoreceptor blockade.