Histological and ultrastructural studies were made on 17 excised canine hearts after 0 (control), 3, 14, 18, and 24 hours of preservation by perfusion with cold filtered plasma. These were correlated with function as measured by an isovolumetric balloon technique. Variations in structure and function were observed within each group of preserved hearts, but function and structure correlated well in individual hearts: when function was excellent, structure was excellent; when function was poor, changes in the sarcoplasmic reticulum and mitochondria were prominent. The presence of marked angulations in the mitochondrial cristae correlated with high dp/dt values. Interstitial edema was present in all preserved hearts. The three control hearts, one 3-hour heart, and two 14-hour hearts showed excellent morphology and function. Of the five 18-hour hearts, function and morphology were excellent in three and barely adequate in one; a severely damaged heart could not be defibrillated. Of the six 24-hour hearts, morphology and function were excellent in three and poor in two; one heart with diffuse mitochondrial damage could not be defibrillated. These studies demonstrate good morphological preservation of canine hearts for 24 hours.