Although respiratory heat loss with cooling of the tracheobronchial mucosa is responsible for the airway obstruction that develops after inhalation of subfreezing air in asthmatics, the mechanism by which airway cooling results in bronchoconstriction is not known. In order to test whether release of endogenous opiate peptides might play a role in mediating this response, asthmatic subjects were studied before and after isocapnic hyperventilation of subfreezing air after the administration of placebo or naloxone, given in a double-blind fashion. Five asthmatic subjects were tested with low-dose (0.8 mg) and 5 with high-dose (10 mg) naloxone given intravenously. Pretreatment with naloxone at either dose did not attenuate the decrease in FVC, FEV1, or FEF25--75 after cold air in comparison with placebo pretreatment. A slightly greater decrease in FEV1 and FEF25--75 after low-dose naloxone than placebo pretreatment can be partially explained by a difference in the temperature achieved during cold air inhalation. We conclude that endogenous opiate peptides are not involved in mediating the bronchoconstrictor response to cold air inhalation in asthmatics.