The choice of oronasal airway was studied in 984 subjects participating in a 10 km run. Subject's age and sex was obtained from entry forms, degree of mouth opening by videotaping the subjects' upper body as they approached the camera, and average running speed from elapsed time. The degree of mouth opening was evaluated independently by three observers. Results indicated that only three (0.3%) of the 917 analyzed subjects breathed nasally during running, 96.3% breathed oronasally, 2.3% opened and closed their mouths rhythmically with breathing, and in 10 subjects it could not be determined whether the mouth was open or closed. No relationship existed between degree of mouth opening and subjects' age, sex or running speed. It was concluded that the incidence of nasal breathing during self-paced endurance running is less frequent than indicated by previous laboratory studies.