The effects of synthetic TRH and methyl TRH (pGlu-N3imMe-His-Pro-NH2) in maternal, fetal and newborn rats were investigated. Colloid droplet (CD) formation in the thyroid cell was enumerated as the index of thyroid hormone secretion induced by TSH. TSH concentration in the serum and pituitary was measured by a homologous radioimmunoassay of rat TSH. 10 fig TRH injected into the maternal tail vein increased the CD in the maternal thyroid from 12⁄25 follicles (control) to 442 μ 20⁄25 (SE) and in the 21 day fetal thyroid from 1⁄25 to 129 μ 12⁄25. 1 μg methyl TRH injected iv to the mother also increased CD in the maternal thyroid and fetal thyroid to 754⁄25 and 76⁄25, respectively. TSH injected to the mother stimulated maternal CD and did not affect the fetal CD. TSH levels in maternal serum at 30 min after administration of 10 μg TRH or 1 μg methyl TRH increased from 20 μU⁄ml to 442 nU⁄ml and 382 μU⁄ml, respectively. Administration of 10 pig TRH to the mother did not cause significant depletion of the pituitary TSH content of the mother or fetus. TRH injected into the umbilical vein or into the fetal abdominal cavity directly did not stimulate fetal CD formation, but did increase maternal CD; both maternal and fetal serum TSH levels rose. 200 mU of bovine TSH injected into the fetus ip caused maximum increase of fetal CD 2 hr later, but did not affect the maternal CD. 10 μg TRH or 10 μg methyl TRH was also given to 1–7-day-old newborn rats ip; colloid droplet formation did not increase in the thyroid of 1- and 2-day-old rats but did increase markedly in those 3 days or older. The results indicate that thyrotropin does not cross the rat placenta and TRH is readily transported across the rat placenta in both directions. When TRH is injected into the pregnant rat, it increases colloid droplet formation in the fetus. Direct administration of TRH to the fetal and newborn rat does not increase colloid droplets until 3 days of age even though serum TSH levels increase. Methyl TRH has-an effect similar to ordinary TRH in the maternal and fetal rat. (Endocrinology94: 1133, 1974)