Abstract: A re-breather that includes an inhalant counterlung (18) and an exhalant counterlung (20), with a mouthpiece (30) disposed therebetween. Gas flows from the counterlung (18) to the mouthpiece (30) for use by the diver, with exhalant from the diver directed to the counterlung (20). Gas forced out of the counterlung (20) is input to a canister/scrubber (10) for processing through a scrubbing material in an interior canister (14). The CO.sub.2 -depleted gas is passed back to the counterlung (18). The counterlung (18) has disposed therein a PPO.sub.2 sensor (68) which is directed toward a control system (66) for controlling a valve (46) to adjust the rate of flow of oxygen from a bottle (40) into the CO.sub.2 -depleted gas exiting the canister/scrubber 10 prior to input to the counterlung (18). The rate is varied to maintain the PPO.sub.2 level at a substantially constant level.