Different types (5) of myocardial protection were employed in this series of 168 patients undergoing aortic valve replacement. Two methods of assessing myocardial preservation were used: cellular biological estimations and quantitative polarization measurements. Both parameters showed that either of 2 methods, continuous perfusion at 32.degree. C with a beating heart or cardioplegic hypothermic arrest, protected the myocardum best. Intermittent perfusion at 30.degree. C with a fibrillating heart was the worst means of preservation. Changes in birefringence, indicative of deteriorating myocardial function, are often detectable before parallel cytochemical changes are apparent.