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The ventilatory response to acute hypoxia is biphasic, with an initial rapid increase followed by a slower decline. In humans, there is evidence that the magnitude of the decline in ventilation is proportional to the size of the initial increase. This study was done to define the role of exogenous opioids in the ventilatory decline seen with prolonged hypoxia.Ten healthy persons were exposed to isocapnic hypoxia for 25 min, followed by 5 min of isocapnic normoxia and 5 min of isocapnic hypoxia. These conditions were repeated during a computer-controlled alfentanil infusion.Serum alfentanil levels were constant among the volunteers (38 +/- 12 ng/ml). Alfentanil decreased both the initial and second acute hypoxic responses (from 1.27 +/- 0.73 to 0.99 +/- 0.39 l [middle dot] min-1 [middle dot] %-1, P < 0.05; and from 0.99 +/- 0.70 to 0.41 +/- 0.29 l [middle dot] min-1 [middle dot] %-1, P < 0.05, respectively). The magnitude of the decrease in ventilation during the 25 min of hypoxia was not changed (10 +/- 3.3 l/min for control; 12.3 +/- 7.5 l/min for alfentanil).Alfentanil reduced the acute ventilatory response to hypoxia. The absolute value of hypoxic ventilatory decline was not increased, but a measure of residual hypoxic ventilatory decline (the ratio of ventilation between the second and first steps into hypoxia) was decreased, which supports the hypothesis that opioids potentiate centrally mediated ventilatory decline.