1. In seven normal subjects the ventilatory responses to progressive isocapnic hypoxia and hyperoxic hypercapnia were measured during rebreathing.

2. During an infusion of somatostatin (10 nmol/mm) the mean hypoxic response decreased by 66% (control: − 1.6 sd 1.2 litres min−1 %−1Sao2: somatostatin: − 0.6 sd 0.7) but the mean hypercapnic response was unchanged (control: 2.0 sd 0.8 litre min−1 mmHg−1; somatostatin: 2.3 sd 1.2). There was no change in resting V˙O2 or V˙CO2 during somatostatin infusion.

3. The opiate antagonist, naloxone (0.1 mg/kg, intravenously), caused little change in either response (mean hypoxic response: − 1.7 sd 1.0 litre min−1 %−1Sao2; mean hypercapnic response: 2.4 sd 0.9 litre min−1 mmHg−1).

4. In five of the subjects the dopamine antagonist, prochlorperazine (10 mg, intravenously), increased the mean hypoxic response by 134% (control: − 1.9 sd 1.4 litres min−1 %−1Sao2; after prochlorperazine: − 3.8 sd 1.6; P < 0.05). The mean hypercapnic response after this drug was also increased (control: 2.1 sd 1.0 litre min−1 mmHg−1; after prochlorperazine: 3.1 sd 1.0) but this change did not achieve significance.

5. The selective effect of somatostatin on the hypoxic response, suggestive of an action on the carotid body, was not inhibited by prior injection of either naloxone or prochlorperazine, and its mode of action remains to be found.

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