Whether endorphins secreted during stressful exercise may play a role in the physiologic response of asthmatics was investigated. Thirteen asthmatic subjects were studied: seven (mean age 16 years +/- 2.8 SEM) had perennial asthma and exercise-induced bronchospasm (EIB), and six (mean age 18 years +/- 2) had mild seasonal asthma without EIB. Each subject performed two matched progressive exercise challenges on consecutive days. Baseline lung function was measured before each challenge and was measured again after exercise at regular intervals for 30 minutes. One minute before exercise each subject received intravenously either i.v. naloxone (0.04 mg/kg), an opiate receptor blocker, or saline, in a double-blind crossover fashion. Heart rate, oxygen consumption, minute ventilation, tidal volume, and arterial oxygen saturation were recorded throughout the tests. During the two challenges these parameters were not significantly different for all 13 patients. The mean percentage reduction in FEV1 after exercise for the seven subjects with EIB was slightly but not significantly less with naloxone (25% +/- 7 SEM) than with placebo (32% +/- 7) during the first 10 minutes after exercise. Naloxone had no obvious effect on EIB or the other parameters measured in association with strenuous exercise in asthmatic patients.