Effect of protein and carbohydrate levels in diet on the response of thyroid hormone metabolism to acute exercise was evaluated in 70 male Wistar rats weighing 90±5 g at the beginning of the experiment. The rats were divided into 3 dietary groups fed isoenergetic diets (about 17 kJ/g) with constant level of fat but different levels of protein )P) and carbohydrate (C): P10 : 10% P and 70% C; P15 : 15% P and 65% C; P20 : 20%P and 60% C. After 3 weeks of rats being accustomed to treadmill running, they were sacrificed at rest and either immediately after 30 and 60 min running at the speed of 20m/min, or after 60 min rest following 60 min running. In rats not subjected to exercise thyroid peroxidase (TPO) activity and plasma thyroxine (T₄) concentrations were directly related to protein intake, plasma triiodothyronine (T₃) to carbohydrate intake. Hepatic iodothyronine 5’-monodeiodinase (5’DI) activity was lower and plasma reverse triiodothyronine (rT₃) was higher in P20 group than in two others. Acute exercise evoked increase in TPO activity in all groups, an increase in 5’DI activity in P10 and P20, and decrease in thyroid hormone plasma concentration: T₄ in P10, T₃ in all groups and rT₃ in P10 and P20. The results indicate that protein and carbohydrate level in diets could influence hepatic 5’DI activity and plasma concentrations of T₄, T₃ and rT₃ both at rest and during exercise, whereas TPO activity seems to be affected by the levels of protein and carbohydrate consumed only at rest.