Alterations of cerebral gas tensions and performance in response to hypoxia, with or without 5% CO2 in the ambient inspirate, were studied in eight conscious rhesus monkeys. The animals were trained to perform a lever press (Sidman) avoidance task. Physiological and performance data were obtained during exposures to normal (21% O2) and hypoxic (12, 10, and 8% O2--all with N2 balance) breathing atmospheres, with or without the addition of 5% CO2. With hypoxia, cerebral PO2 and PCO2 declined steadily, but a significant performance decrement was noted only at the 8% O2 level. With the addition of 5% CO2 to the inspired atmospheres, cerebral PO2 was elevated relatively but still declined as hypoxia intensified. Cerebral PCO2 and the avoidance task performance were sustained at near baseline values with the 5% CO2 inspirate.