Abstract: An oxygen consumption monitor system suitable for use in measuring oxygen uptake in neonates wherein the system includes a check valve system in combination with a high flow, positive-end-expiratory-pressure respirator for supplying a high volume, controlled flow of oxygen through a flow circuit which includes (1) a primary flow line for carrying most of the oxygen supply through a respirator circuit maintained at a positive pressure, and (2) a secondary outlet communicating from said primary flow line through the check valve combination to permit inspiration of oxygen from the respirator circuit and expiration to a separate chamber for measurement of oxygen consumption. Minimal deadspace is developed in the check valve combination to insure fresh oxygen supply with each inspiration by the neonate. The use of opposing oneway valves at inlet and outlet locations of the valve combination permits use of a single opening for coupling to an endotrachael tube.