Membrane oxygenators reputedly preserve erythrocytes, platelets, kidneys, brain and lungs better than bubble oxygenators; preservation of ventricular function by the two types of oxygenator is compared in isolated hearts, extremely sensitive to imperfections of perfusion, by isovolumic tests which are simple and accurate, especially for evaluating compliance. Canine hearts were perfused for three hours with either disposable bubble (Temptrol) or membrane (Lande-Edwards) oxygenators. Values at a standard point on regression slopes os isovolumic contractile force, velocity and compliance (volume) against end-diastolic pressure were used to express final functions as percentages of initial ones. Terminal proportional ventricular weight was an index of oedema. The final mean percentages with standard error measurements of initial values for the 12 hearts perfused on buble oxygenators and the 10 on membrane oxygenators were, respectively: 97 +/- 11.5% and 87 +/- 10.7% for contractile force, 117 +/- 23.1% and 88 +/- 10.44% for contractile velocity, and 97.2 +/- 8.48% and 117.3 +/- 12.5% for ventricular compliance, which was the function nearest to showing a significant difference with P less than 0.1. There was no significant difference in weights. This membrane oxygenator, as cheap and simple as conventional ones, probably has similar advantages for the myocardium as for other tissues.