The activity of the thyroid gland was estimated in a group of lactating [human] mothers and their suckling infants as well as in a group of early weaned infants. In both groups of infants the same level of thyroxine and triiodothyronine was found, but it was higher than that in mothers. The binding properties of specific plasma proteins and free thyroxine index were the same in both groups of infants. The level of TSH [thyrotropin] in serum of suckling infants (4.7 .+-. 0.5 .mu.U [microunits]/ml) was significantly higher than that in weaned infants (3.0 .+-. 0.3 .mu.U/ml). The level in mothers (4.0 .+-. 0.3 .mu.U/ml) did not differ significantly from that in both groups of infants. A positive correlation was found between the level of TSH in lactating mothers and their suckling infants. The regulation of thyroid activity in infants may be affected by suckling.