Cardiopulmonary bypass (CPB) devices replace transiently the function of both heart and lungs, allowing the investigator to work safely on a stopped heart. Although this technology has greatly improved since its first applications in the early 1950's there is still no definitive rat CPB model, various experiments reporting drawbacks like pulmonary edema, large priming volumes,etc. We present a new oxygenator that can be used in cardiopulmonary bypass experiments in rats, simple in design and efficient in function,in which the process of blood oxygenation takes place in a vertical cylinder filled up with air or oxygen, with blood being spread onto the wall and then trickling down in a thin layer that facilitates the oxygen transfer. The oxygenation is efficient, the pO2 reaches levels of almost 150 mmHg (physiological level is around 100 mmHg) in conditions of oxygen saturation of over 99% (normal levels 95%-98%).