The study reported here extends investigation on the pituitary thyroid axis in newborn infants, including the assay of plasma immunoreactive TSH levels at different intervals after delivery. Blood samples were collected at birth and after 30, 60, 120 minutes, 6, 24 and 48 hours. Plasma TRH levels were also estimated in normal adult subjects and pregnant women. No significant difference was observed with regard to sex, pregnancy or age, except for a marked increase in newborn infants after delivery. Plasma TRH values, already moderately high at birth (mean 46 pg/ml, range 34-57) reached rapidly a peak of 78 pg/ml (range 60-93) 30 minutes after delivery, decreased rapidly between 30 minutes and 2 hours post-partum, then fell gradually to normal range at 24 hours. A comparison of plasma TRH and TSH levels measured simultaneously suggests that the acute TSH surge at delivery is mediated by TRH secretion.