Very few sequential studies of plasma thyroxine (T4) levels from birth are available in any species. In human infants, the serum protein-bound iodine (PBI) rises significantly during the first week of neonatal life (Danowski, Johnston, Price, McKelvy, Stevenson & McCluskey, 1951). There then follows a gradual decrease in PBI until by the 6th to 12th week it is below the level at birth. In the calf, the plasma T4 level is elevated in the first 3 hr. of extra-uterine life when it is 12·8 ± 0·6 μg./100 ml. (Nathanielsz, 1969). There then follows a rapid fall in plasma T4 to a level of 4·2 ± 0·6 μg./100 ml. on day 4. A secondary rise occurs with a peak value of 5·7 ± 1·2 μg./100 ml. on day 21. This rise appears to precede the onset of steady growth in the calf which occurs at day 20–25.