Measurements of serum thyrotropin (TSH) concentrations were conducted in maternal and fetal blood during labor and delivery and the early postnatal and neonatal periods. Mean TSH concentration was significantly higher in cord blood (9.5 muU/ml) than maternal blood (3.9 muU/ml), a finding suggesting a fetal-maternal TSH gradient at term. Serum TSH concentration in the newborn increased rapidly to mean levels of 60 muU/ml at 10 min and 86 muU/ml at 30 min of age. Between 30 min and 3-4 hr serum levels decreased rapidly, then fell more gradually to a mean concentration of 13 muU/ml at 48 hr. The half-time of the decrease in serum TSH concentration between 30 and 90 min was 77 min, a value slightly greater than the half-time of disappearance of radioiodinated TSH measured in adults. This indicates that the early high rate of TSH secretion in the newborn ceases by 30 min, and that the rapid rise and fall in serum TSH concentrations may represent release of stored pituitary TSH. A more chronic TSH hypersecretion persisted throughout the first 24-48 hr of extrauterine life. Measurements of serum PBI concentrations were conducted in a separate group of maternal and cord blood samples and in the newborn infants during the first 48 hr of extrauterine life to relate the TSH and serum hormonal iodine concentration changes. Serum protein-bound iodine (PBI) concentrations were similar in maternal and cord blood, increased significantly by 4 hr of age in the newborn, and peaked at about 24 hr, presumably in response to the TSH hypersecretion. The pattern of TSH hypersecretion was similar in infants delivered vaginally and by caesarean section. Maternal serum TSH concentrations were stable throughout the perinatal period. Warming the infants at 99-103 degrees F during the first 3 hr of life did not prevent the early, acute release of thyrotropin. Cooling of warm infants at room temperature (72-78 degrees F) between 3 and 4 hr resulted in a decrease in mean rectal temperature of 3.3 degrees F and produced a significant increment in serum TSH concentration. These data suggest that the mechanism of the early, acute release of thyrotropin in the newborn may involve a potent stimulus other than cooling. However, the increase in serum TSH stimulated by delayed (3-4 hr) cooling indicates that neonatal hyperthyroxinemia is, at least, augmented by extrauterine cooling. Thus, cold exposure is capable of increasing TSH secretion in humans.