The activity of the thyroid gland of the newborn rat is influenced by the treatment of TSH. Shortly after hormone injection, its effects can be detected in the synthesis and storage of the colloid. In a longer term (30 min and 60 min) the morphological signs of resorption and secretion are evident. At this early age the cellular compartments involved in the secretory pathway of the cells are still not complete in maturation (less developed Golgi complex, missing secretory vesicles and micropinocytotic vesicles). According to our results, even TSH administration cannot influence this picture in the short term. Our work supports the intracellular way of folliculogenesis, and the TSH action as a promoter in this way.