The neonatal development of the rat thyrotrope was studied in sections of male rat pituitaries with stereometric techniques on cells that had been identified cytochemically with antirat TSHβ and either the peroxidase-antiperoxidase technique or the avidin-biotin-peroxidase complex technique. The TSH cells were poorly granulated at 2 days of age, and the hormone was stored in pleomorphic saccules or granules. In adult rats, thyrotropes were 4–6% of the pituitary cell population. In the neonatal rat pituitary, the thyrotropes proliferated in clusters, and the percentage of stained cells increased to 18.8% by 9 days of age. The TSH cell percentages then decreased, and the clusters dispersed. The 15-day-old rat pituitaries contained 5% thyrotropes. The TSH cells matured morphologically during the first 10 days of development, and by 7 days, they contained the small granules that are typical of the thyrotrope. After that time, the thyrotropes enlarged and partially degranulated, and TSH/3 was stored in some of the large pleomorphic saccules in the cytoplasm. Our studies show that the greater sensitivity to TRH of the thyrotropes in the 5- to 10-day-old rat may be due to the relatively high numbers of thyrotropes in the gland. We suggest that the 9- to 10-day-old male rat pituitary might be useful for a study of separated purified thyrotropes because of its relatively high content of TSH cells.