This invention relates to a portable rebreathing system, said portable rebreathing system comprising a breathing mask (4), a carbon dioxide scrubber (3), a counter lung (2), ambient air ports and an oxygen supply port (5), with the breathing mask (4) connectable with a mask connector to a common valve housing (X) containing said carbon dioxide scrubber (3), ambient air port and oxygen supply port, and with the counter lung (2) connectable with a counter lung connector to the common housing.The main features of the invention are two one way check valves close to breathing mask connects to an exhale passage and an inhale passage respectively in a rebreathing mode. A small common valve housing (X) with control valves open a rebreathing circuit to the counter lung when pressure from an oxygen supply source is applied on the control valves, and when pressure drops open a breathing circuit to ambient air. Supply of fresh oxygen is added into the rebreathing circuit directly upstream of or at the inhale check valve.This design utilizes the oxygen supply efficiently with minimum oxygen losses.
