Eight adult patients with sleep apnea were studied to evaluate the ventilatory and cardiac effects of hyperoxia on an acute basis. Five patients then used low-flow nighttime oxygen for 30 to 90 days. The results of the acute study showed that for 30-min study periods, the total number of apneas and per cent apnea time (duration of apneas divided by sleep time) decreased significantly from the room air to the oxygen period (207 to 68, p less than 0.05; 41.1 +/- 18.3% SD to 20.5 +/- 14.4%, p less than 0.05, respectively). Also, the apnea-associated slowing in heart rate is blocked by the supplemental oxygen. Three patients receiving oxygen at home decreased their per cent apnea time by greater than 60%. These patients also responded to the acute administration of hyperoxia by a decrease in apnea time greater than 60%. One patient prolonged the apnea time, and one had a minimal positive response, both again reflecting their acute studies. These data suggested that the severe hypoxemia that develops during an apnea in this syndrome has a central ventilatory effect that propagates the apneas and is significantly reversed by hyperoxia.