The breathing patterns after voluntary hyperventilation were determined in 14 healthy young subjects under four different conditions: (1) normoxia, (2) hyperoxia, (3) hypoxia, and (4) sudden administration of oxygen against a background of hypoxia to evaluate the effect of hypoxia on the pattern of the posthyperventilation breathing. Under hypoxia the ventilation in the immediate posthyperventilation period was depressed although no decreased ventilation was observed under hyperoxia or normoxia in this period. Sudden administration of oxygen depressed further the decreased ventilation in the posthyperventilation period.