In the acute phase of acute myocardial infarction (3-8 h after onset of symptoms) an early transient increase in the creatine concentration of serum, saliva, and especially of urine can be observed. Due to the renal threshold, urine values give a much better discrimination between infarction patients and controls than do serum determination. In some patients secondary peaks of serum and urine creatine concentrations can be seen about 24-36 h after hospital admission. Intramuscular injections of 5.0 mL of a saline solution and muscular trauma interfere with the test, but with angina pectoris interference is absent or limited. Creatine leakage from myocardium is insufficient to explain the observed creatinuria in infarctions, and intact extra-cardiac tissues are believed to be involved in creatine release.