Measurements of serum thyroxine (T4), thyroxine binding globulin (TBG), T3 [triiodothyronine] talc uptake (T3U), 3,5,3''-triiodothyronine (T3), 3,3'',5''-T3 (reverse T3 or rT3) and thyrotropin (TSH) concentrations were conducted in 195 healthy children and adolescents 1-15 yr of age. All were of normal height and weight, had thyroid glands normal to palpation and none were taking medication. Caucasian, Asian and black children were included in the study. Regression analyses were made for each variable against race, sex and age. There were no significant racial or sex differences noted for any of the variables studied. There were no age-related changes in serum TSH or T3U. Linear regression of serum T4, T3 and TBG levels showed that each decreased significantly (P < 0.001 by F-test) between 1-15 yr; the decrease in mean concentration approximated 30% in each instance. The decrease in serum T4 and T3 concentrations with age are largely accountable on the basis of decreasing TBG levels. Decreasing free T4 index and free T3 index values suggest a progressive decrease in free T4 and free T3 levels with age. There are earlier data indicating a progressive decrease in T4 production with age, but whether the decreasing free T3 index is due to decreased T3 production is not known. Mean serum rT3 concentrations and free rT3 index values increased slightly but significantly with age. Whether the increase in serum rT3 level with age reflects increased production and/or decreased metabolic clearance is not known.