Twenty-eight dogs were subjected to 90 minutes of hypothermic (30 degrees C) cardiopulmonary bypass with moderate hemodilution. In 6 dogs the heart was vented and beating for 60 minutes. Eight dogs underwent ventricular fibrillation with coronary perfusion (VF + CP). In 14 dogs the aorta was cross-clamped for 60 minutes while the myocardium was protected by local cardiac hypothermia (ICA + LCH). Eighteen animals survived. Hemodynamic studies at seven weeks revealed no major differences among the three groups. At postmortem examination, no gross scarring was noted in any heart. Microscopical examination of 14 hearts was completely-normal. In the VF + CP group, 2 hearts had isolated microscopical scars. Similar linear subendocardial scars (less than or equal to 1.5 X 0.5 mm) were noted in 2 hearts subjected to ICA + LCH. Survival after 60 minutes of VF + CP or ICA + LCH did not result in long-term morphological injury to or functional impairment of the myocardium.