Trimethyllysine(TML) is in vivo released when the methylated proteins are subjected to metabolic breakdown and may serve as a precursor for the biosynthesis of carnitine. It was also demonstrated that high concentration of free TML was observed in red blood cell. Therefore, in order to study the functions of TML in RBC we determined the concentration of TML and carnitine in RBC of hyperthyroid and hypothyroid rats and the following results were observed. 1)In hyperthyroid rats, the concentrations of TML, free carnitine and acylcarnitine in RBC were increased. 2) In hypothyroid rats, the concentrations of TML and acylcarnitine were increased but free carnitine was sharply decreased. 3) In diabetic rats, TML and free carnitine both were inclined to decrease, but not significant statistically. 4) In fasting rats, TML was not changed but free carnitine was sharply decreased. These results suggest that TML in RBC is not directly related to biosynthesis of carnitine.