A mouthpiece for a rebreather has a tubular housing having opposed inhale and exhale ends, a mouth port, and a discharge port. Supported for movement within a bore of the housing is a valve assembly which is magnetically biased toward a valve-closed position preventing air in an air space of the bore from moving to the exhale end and the discharge port. As a diver exhales into the mouth port, the increase in air pressure of the air space causes the valve assembly to assume a valve-open position, exposing a transverse channel extending between the air space and the discharge port, and a recirculation air channel extending between the air space and the exhale end. A portion of the exhaled air is exhausted to the ambient environment through the discharge port, while the remainder exits the mouthpiece at the exhale end for recirculation through the rebreather.