The urinary excretion of N tau-methylhistidine (3-methylhistidine: 3-Mehis), an index of the rate of myofibrillar protein catabolism, was determined in intact and thyroid ectomized mature male rats, receiving intraperitoneally either vehicle (0.9% NaCl) or thyroxine (T4) replacement (2 microgram/100 g body weight/day) during 20 days. Rats were fed either an adequate control of a low-protein low-energy diet. In addition, body weight changes and food intake were recorded throughout the experiment. At the end of the 20-day period, livers and several muscles from hind limbs were excised and weighed. A sample of blood was then taken for serum insulin, triiodothyronine (T3) and thyroid-stimulating hormone (TSH) determination. As compared to the well-nourished animals, a significant (p less than 0.05) reduction in the rate of growth, food intake, 3-Mehis and serum insulin and T3 concentrations was observed in the rats fed the low-protein low-energy diet. In both dietary groups, thyroidectomy increased serum TSH levels and tended to reduce 3-Mehis output and liver and muscle sizes, although there was a different response according to the type of muscle excised. T4 replacement improved growth and restored T3 levels, especially in the well-fed animals, but it failed to restore either serum insulin concentrations or 3-Mehis output in either dietary groups. In conclusion, both thyroidectomy and protein-calorie malnutrition reduced the rate of myofibrillar protein breakdown in the mature rat and T4 replacement had no effect in restoring the normal range of myofibrillar protein degradation.