The hypothalmic content of thyrotropin-releasing hormone (TRH), the pituitary concentration of thyroid-stimulating hormone (TSH), and the serum concentrations of TSH, thyroxine (T4), and triiodothyronine (T3) have been determined at different intervals during the first 50 days following birth in the rat. From a minimum concentration of 1 pg/mug protein at birth, the hypothalamic concentration of TRH increased to a maximum of 5 to 6 pg between 16 and 28 days of age. Serum and pituitary TSH concentrations increased to maximum levels by the end of the first post-natal week; the elevated hormone levels were then maintained to the end of the third post-natal week. Circulating thyroid hormone concentrations were very low at birth. T4 increased rapidly between days 4 and 16 to reach a peak concentration of 6 mug/100 ml, while T3 followed a parallel pattern with a peak concentration of 108 ng/100 ml obtained only at day 28. These data indicate that, in the rat, components of the hypothalamic-pituitary-thyroid axis develop simultaneously during the post-natal period.