We have used a physical model of the thermal conditions of open-heart surgery to study sources of heat input to the heart during local cardiac cooling. Pulmonary and systemic venous return entering the cardiac chambers were the most important sources of heat to the hypothermic heart. In 5 excised hearts, venous return of 100 ml per minute or more entering the left atrium and left ventricle increased mean septal temperature significantly from 8 +/- 1 degrees C to 16 +/- 1 degree C (p less than 0.01). When venous return passed through the right side of the heart and then the left side, it increased mean septal temperature significantly from 7 +/- 0.5 degrees C to 23 +/- 1 degree C (p less than 0.001). Conduction of heat through the pericardium, heat radiation from standard operating room lights, and heat uptake from room air had relatively minor effect and produced no significant increase in myocardial temperature provided all surfaces of the ventricles were irrigated with cold saline. The hypothermic heart can be isolated from heat input by individual caval cannulation, low bypass perfusion rate, systemic cooling to 30 degrees C, and irrigating all surfaces of the ventricles with cold saline.