The creatinuria after musculoskeletal injuries was studied in 31 patients. In addition to creatine the urinary outputs of creatinine, total nitrogen, and 3-methylhistidine were studied. Plasma creatine, creatinine, and guanidinoacetate concentrations were measured in some patients. In rats the effect of bilateral hind-limb ischemia was observed on the urine outputs of creatine and creatinine, and on the creatine, protein, and DNA contents of the hind limbs and carcass.In man the creatinuria was positively related to the severity of the injury and this relationship was stronger than with the urinary output of either creatinine or total nitrogen. In the rat creatinuria was related to the duration of the limb ischemia. The mechanism of the creatinuria is not known but the experiments on rats showed part of the excess creatine to be derived from the damaged muscle and excreted shortly after the injury, and part from the undamaged muscle of the carcass which provided the delayed excretion. It is suggested that creatinuria could be used as an indicator of the post-traumatic 'flow' phase.