The influence of age on the kinetics of thyroxine (T4) and triiodothyronine (T3), and on peripheral conversion of T4 to T3, has been studied, comparing a group of euthyroid, healthy, elderly individuals to a group of young individuals. A monocompartment model was adopted for the kinetics of T4, while a bicompartment model was used for T3. No significant difference (p greater than 0.05) could be found in any of the kinetic parameters of T4 between the elderly and young groups. However, the following significant (p less than 0.05) changes were noted for T3: an increased disposition constant in the phase of slow distribution (beta), shortening of the half-life of this phase (T 1/2 beta ), elevation of the central distribution volume Vc and the elimination constant K23, reduction of the K12: K23 ratio, and increased plasma metabolic clearance and hormonal daily turnover. No significant differences (p greater than 0.05) were observed for the percent turnover of T4 converted to T3.