Reported values of the concentration of thyroid hormones in milk vary substantially. There are some doubts on the specificity of methods used for their estimation. We aimed, therefore, to study the effects of mother milk on thyroid function parameters in sucklings as well as at the effect of milk secretion on maternal thyroid. According to Fukuda et al. [1980] the lactation induces a hypothyroid state in the rat (high TSH, low thyroid hormones in maternal plasma). In our study the weaning of 18-day old rats resulted in gradual decrease of T4 in suckling plasma through 24 h followed with a transient thyroid activation. Thyroidectomy of lactating rats resulted in a transient decrease of T4 in sucklings. The difference in mother milk intake achieved by different litter size also affected the thyroid function of sucklings. The pups from small size litters (more milk consumption) had low thyroid secretion rate and low plasma TSH, while the level of T4 in plasma was high under a higher growth rate and accelerated overall maturation. All these results suggest an exogenous intake of thyroid hormones in suckling rat. We found that lactation in women resulted in lower T4 and higher TSH in plasma as compared to women who had interrupted their lactation. In the next part of our study more than 80 infants were longitudinally followed and sampled at birth (cord blood), at 2, 6 and 10 weeks and at 4, 6, 9 and 12 months. Only moderate differences were found at 4 months (high rT3) in breast fed infants and 9 at months (high T3 in plasma of infants weaned during the first week of life) when the data were evaluated according to the type of feeding. Thyroid activity of human milk was tested by feeding thyroidectomized rats with a diet containing human milk. Although plasma TSH was affected by such a diet, we did not detect any T4 and T3 in plasma of thyroidectomized rats which probably refects low calculated intake of hormones with the diet. We conclude that the lactation affects the thyroid economy of maternal organisms. Maternal milk definitely affects thyroid function parameters in the rat, while the effect on infant thyroid is more difficult to demonstrate.