To verify the hypothesis that the rate of proteolysis is increased during ischemia, slices of ischemic and non-ischemic tissue from dog heart were incubated in either aerobic or anaerobic media. Tyrosine production, which is an index of the rate of protein breakdown, was measured by a spectrofluorimetric method. Preliminary results indicate that proteolysis is decreased by about 50% in both one and three-hour-ischemic tissue. Leupeptin, an inhibitor of thiol-proteases, decreased proteolysis by about 25% at 50 microns. Furthermore, experiments are in progress to rule out the possibility that tyrosine production depends on the type of medium used.