Normal term newborn infants have been studied during the first four hours after birth. Cord blood and brachial venous blood samples were collected for measurement of serum prolactin (n=15; 11 vaginal delivery; four cesarean section delivery), thyrotropin (n=8), growth hormone (n=7), tri-iodothyronine (n=7) and thyroxine (n=7). The mean cord serum TSH concentration was 12±2 μU/ml and peaked in all infants 30 minutes after birth (67±7 μU/ml). Blood PRL also increased in all of the 15 infants studied; the mean cord blood value of 165±15 ng/ml increased significantly to a mean of 214±22 ng/ml at 30 minutes and 204±17 ng/ml at 60 minutes. The peak blood PRL concentration was observed at 30 minutes in eight newborn infants, at 60 minutes in three, and at 120 minutes in four. Mean serum GH concentrations were statistically similar during the first four hours but there was a tendency for serum levels to fall somewhat during the first hour after birth. The mean cord serum T3 concentration (47±5 ng/ml) increased significantly to a peak value of 227±38 ng/dl at 120 minutes. The increase in T4 from a mean of 15.3±1.2 μg/dl to a mean of 17.9±2.6 μg/dl at four hours was less dramatic. These data are consistent with the possibility that the same mechanism responsible for the neonatal TSH surge may be responsible for a significant but lesser PRL surge during the early minutes after birth.