The hypothalamic content of thyrotropin-releasing hormone (TRH), the pituitary concentrationof thyroid-stimulating hormone (TSH),and the serum concentrations of TSH, thyroxine(T₄), and triiodothyronine (T₃) have been determinedat different intervals during the first 50days following birth in the rat. From a minimumconcentration of 1 pg/μg protein at birth, the hypothalamicconcentration of TRH increased to amaximum of 5 to 6 pg between 16 and 28 daysof age. Serum and pituitary TSH concentrationsincreased to maximum levels by the end qf thefirst post-natal week; the elevated hormone levelswere then maintained to the end of the thirdpost-natal week. Circulating thyroid hormone concentrationswere very low at birth. T₄ increasedrapidly between days 4 and 16 to reach a peakconcentration of 6 μg/100 ml, while T₃ followeda parallel pattern with a peak concentration of108 ng/100 ml obtained only at day 28. Thesedata indicate that, in the rat, components of thehypothalamic-pituitary-thyroid axis develop simultaneouslyduring the post-natal period. (Endocrinology97: 1321, 1975)