Under certain circumstances a person swimming under water may lose consciousness. Eight incidents here described indicate that hyperventilation before breath-holding and exercise may delay the onset of the urge to breathe. Before the partial pressure of CO2 increases significantly, the O2 may decrease to a degree incompatible with high-level cerebral function. In 5 cases of drowning also reported, this chain of events is likely to have occurred. Discussion of the details suggests that certain preventive steps can be taken without discouraging swimmers from learning to handle themselves under water.