Apneic thresholds in man anesthetized with diethyl ether, halothane, and Forane were compared. Assisted ventilation to apnea lowered Paco2by an average of 4.6 torr from Paco2during spontaneous respiration. There was no difference between anesthetics or depths of anesthesia in the reduction of Paco2needed to achieve apnea Apneic thresholds were lower in subjects anesthetized with ether than in those anesthetized with halothane or Forane because Paco2with spontaneous respiration was lower with ether than with either of the other two anesthetics. During ether anesthesia apneic threshold did not change significantly as end-tidal ether concentration was increased from 3.0 to 6.0 per cent. During halothane anesthesia apneic threshold and Paco2during spontaneous ventilation increased by 9 torr as end-tidal halothane increased from 1.0 to 1.5 per cent. We conclude from these results that: 1) assisted ventilation can produce only a minor reduction in Paco2; 2) ether does not stimulate respiration independent of CO2.