Summary: Sprague-Dawley rats were sacrificed by decapitation at 5, 7, 12, 14, 22, 26, 32, and 40 days of age. Adult animals (175 to 225 g) were also studied. Serum-free thyroxine (FT4) concentrations rose rapidly between 5 and 12 days to levels similar to adult concentrations, whereas the percentage of FT4 was relatively high between 5 and 12 days before declining to adult values by 14 days. Serum-free triiodothyronine (FT3) concentrations rose progressively to attain peak concentrations at 26 days and subsequently declined to adult levels by 40 days. The percentage FT3 rose in parallel with the FT3 concentrations to peak values at 22 to 26 days before declining to adult levels. FT4/thyroid-stimulating hormone (TSH) and FT3/TSH ratios increased progressively through 22 days of age in parallel with the FT3/FT4 ratio. These data indicate that free thyroid hormone concentrations follow essentially the same developmental profile as do total thyroid hormone concentrations. Progressive maturation of the negative feedback control mechanism for the pituitary-thyroid axis, as assessed by the FT4/TSH and FT3/TSH ratios, occurs through 14 days. However, the continued rise in FT3 concentrations, FT3/TSH, and FT3/FT4 ratios through 26 days suggests a further resetting of the setpoint of the pituitary-thyroid axis possibly related to the stress of weaning. Speculation: Maturation of the pituitary-thyroid axis in the rat is principally a postnatal phenomenon. Careful study of the developmental profile of this axis under physiologic and pathologic states may afford considerable insight into the ontogenesis of the pituitary-thyroid axis in the human fetus.