In adult humans the ventilatory response to sustained hypoxia is biphasic, characterized by an early increase followed by a decline to an intermediate plateau. Recently, we have shown that this decrease in hypoxic sensitivity is long lasting, because up to 1 h of room air breathing is required for complete recovery of the initial hypoxic response (J. Appl. Physiol. 64: 521-528, 1988). It is not known whether this posthypoxia decrease in ventilatory response is general or specific only to hypoxic stimuli. We therefore examined responses to CO2 before and after hypoxia. The ventilatory response to 5 min of normoxic CO2 breathing was evaluated in eight normal adults on 2 days: 5 min before and after 25 min of normocapnic hypoxia (arterial O2 saturation +/- 80%) and 5 min before and after 25 min of room air breathing (control day). During hypoxia, ventilation (VI), after an initial increase, declined significantly. At the end of hypoxia, abrupt exposure to room air transiently dropped VI to values that were significantly below base line. On each experimental day, the first and second exposure to CO2 increased ventilation by a similar amount, averaging, respectively, 8.46 +/- 0.9 and 8.84 +/- 0.92 (SE) l/min on the hypoxic day and 8.24 +/- 0.96 and 7.65 +/- 0.94 l/min on the control day. All the hypercapnic increases of VI were accomplished through similar changes of breathing pattern with similar time courses. We conclude that sustained hypoxia does not affect the ventilatory response to CO2, but it selectively depresses hypoxic sensitivity.