The object of this experiment was to examine whether the administration of clenbuterol exerts clenbuterol's repartitioning effects in rats with hypothyroidism induced by the oral administration of propylthiouracil (PTU). Male Wistar rats aged 5 weeks were divided into 4 groups: control, PTU administrated (PTU), clenbuterol administrated (CL), and PTU plus clenbuterol administrated (PTU/CL) groups. Rats were raised for 7 weeks at 26 degrees C and given 13 g of diet every day. The PTU and PTU/CL groups were fed a basal diet containing PTU at the level of 30 mg/kg diet throughout the experimental period. Clenbuterol was added to the diet of the CL and PTU/CL groups at the dose of 0.1 ppm from the 3rd week. Serum thyroxine concentrations of rats were significantly lower in rats in the PTU and PTU/CL groups than those in the control and CL groups. Thus, the administration of PTU succeeded to induce hypothyroidism. The clenbuterol administration seemed to exert its repartitioning effects in euthyroid rats, while the administration neither increased body protein nor decreased body fat in hypothyroid rats. This result therefore suggests that the effects of clenbuterol on the alteration of body composition may vary with the level of thyroid activity in rats.