In twenty-nine patients suffering from acute myocardial infarction changes of concentrations in plasma of creatine kinase, aspartate aminotransferase and lactate dehydrogenase were monitored 1 week following onset of infarction. The temporal characteristics of enzyme changes described are (i) the time lag from onset of chest pain until increasing enzyme concentrations occur, (ii) the time of maximum concentrations, and (iii) the period during which enzyme is released into blood. Three estimates for the extent of the infarct (i) the peak value of the plasma enzyme curves,(ii the value of the cumulated plasma curves, and (iii) the size of the infarct in grams of necrotic myocardial tissue were, as expected, closely correlated. It is concluded, that the three types of quantification of infarct size are of almost identical value for clinical, prognostic usage, From a pathophysiological point of view they are of limited interest being based on assumptions that are either unlikely to occur or cannot be tested in man.