Using rats pre-labelled by [14C]creatine injection, the mechanism of creatinuria induced by experimental hyperthyroidism during a 20 day treatment period has been examined. The excess creatine excretion is the result of increased release of creatine from muscle and increased excretion of creatine synthesized de novo. The increased excretion of newly synthesized creatine in hyperthyroidism appears to occur because of decreased creatine uptake by muscle, an increased rate of creatine synthesis, or a combination of both factors. The radioactive creatine content of hyperthyroid skeletal muscle was reduced at the time of death indicating an increased rated of creatine loss during the treatment period. In spite of the increased loss of labelled creatine, the specific activity of creatine obtained from hyperthyroid skeletal muscle was higher than control. This finding is probably due to decreased uptake of unlabelled creatine by hyperthyroid skeletal muscle during the treatment period. In contrast to skeletal muscle, the radioactive creatine content of hyperthyroid myocardium was higher than control at the time of death suggesting a decreased rate of creatine loss during the treatment period.