A practical mechanical heart and lung is described in which glass syringes are used to replace the ventricles. The pistons of the syringes are operated by a special cam rather than a simple crank. Thus the volume changes in the syringes are made to follow those of the mammalian ventricles. The heart rate and stroke output may be varied at will and read at any moment. The artificial lung consists of a large percolator contained in a water bath. Oxygenation of the blood is maintained mainly by passing it through an aggregation of oxygen bubbles rather than by passing the oxygen through the blood. Connections of the apparatus to the animal are such that the blood is withdrawn from the right auricle and replaced into the aorta at its base. A technique has been developed whereby only 20 sec. are required to change from the animal’s heart and lungs over to the mechanical apparatus. Thus the organs, including the brain, suffer little hypoxia during the change.