ABSTRACTThin freehand slices of left ventricular papillary muscle of the dog, exhibit good cell volume regulation when incubated at 37° for 60 minutes in oxygenated Krebs Ringer phosphate solution. The fine structure of the cells is maintained throughout the incubation. Thisin vitrosystem was developed in order to test the capacity of myocardial cells irreversibly injured by 60 minutes of ischemia to maintain cell volume. The results showed that irreversibly damaged cells were unable to maintain volume. They swelled markedly, lost Mg2+and K+and exhibited structural defects in the plasma membrane of the sarcolemma. These observations establish that loss of cell volume regulation is one of the early events associated with the development of irreversibility in severe myocardial ischemic injury