Several incidents of losing consciousness while swimming underwater are known to the author. Experiments were designed to find out how this could happen. Four types of breath holding were executed: a) at rest, b) after hyperventilation, c) during mild exercise, and d) after hyperventilation and during exercise. At the breaking point the subject made a maximal expiration, and the end tidal air was analyzed for O2 and CO2. It was found that when the breaking point was reached, the Pco2 was higher and the Po2 lower during exercise than at rest. The lowest Po2 was observed after the subject had exercised following hyperventilation: the Po2 was 34 mm or below in 4 of the 12 subjects, a degree of hypoxia often associated with unconsciousness. Other experiments including underwater swimming support the conclusion that loss of consciousness after hyperventilation and during exercise is possible and is probably due to hypoxia. Submitted on December 30, 1960