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Isolated rat hearts were perfused at 37 C on a double reservoir, nonrecirculating Langendorff apparatus. For aerobic perfusion, Krebs-Henseleit medium containing 10 mM glucose was gassed with 95% O2-5% CO2; for anoxic perfusion, glucose was replaced with 10 mM mannitol, a nonmetabolizable substrate, and the medium was equilibrated with 95% N2-5% CO2. Heart effluent was serially collected during perfusion for creatine phosphokinase activity (CPK) analysis. Fixation was with 1% glutaraldehyde for morphologic studies. With aerobic control perfusion, hearts continued contracting, released no CPK, and were morphologically normal by light and electron microscopy examination after 120 minutes. With anoxic perfusion, contractions soon ceased, and by 60 minutes a sustained slow release of CPK was first observed. By electron microscopy, cells at 60 or 90 minutes were swollen and contained amorphous matrix densities in mitochondria; a few cells showed breaks in cell plasma membrane. When anoxic hearts were challenged with reoxygenation, there was a sudden change in color to a pale opaque appearance, CPK was rapidly released, and there was massive cellular swelling. By electron microscopy, damaged cells showed contraction bands, clumping of nuclear chromatin, both amorphous densities and granular dense bodies in mitochondria, and prominent disruptions of cell plasma membranes. The number of damaged cells observed increased as a linear function of time between 30 and 55 minutes of anoxia. The results show that anoxic perfusion in vitro produces irreversible myocardial injury and that this injury is closely associated with loss of cell volume control, release of intracellular enzymes, and striking structural defects in the plasma membrane of the sarcolemma. Reoxygenation accelerates the development of lesions in irreversibly injured cells but protects reversibly injured cells.