1. Endogenous opioids have been implicated in the control of breathing in neonates, but their role in ventilatory control in adults remains unclear.

2. We studied the relationship between circulating immunoreactive β-endorphin and the ventilatory and mouth occlusion pressure responses to hypercapnia in 12 healthy male subjects. In addition, we examined the effect of repetitive hypercapnia on plasma β-endorphin and Cortisol levels.

3. A weak but significant negative relationship between the ventilatory response to hypercapnia and basal plasma β-endorphin levels was observed (r = −0.35, P < 0.01). A similar negative relationship was noted between mouth occlusion pressure response to hypercapnia and basal plasma β-endorphin levels (r = −0.36, P < 0.01).

4. Repetitive hypercapnia prevented the fall in plasma Cortisol that occurred under control conditions (P < 0.02) but had no effect on plasma β-endorphin.

5. We conclude that plasma β-endorphin may play a role in the central chemical control of breathing in man.

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