The changes in plasma thyroid hormone concentration were studied during exchange transfusion performed for haemolytic disease. 24 transfusions were performed using blood preserved with acid-citrate and dextrose and in 11 cases 10 or 50 mug glucagon was added to the donor blood. Plasma tri-iodothyronine (T3), thyroxine (T4), thyrotropin (TSH), thyroid hormone binding capacity, and free thyroxine index were measured in the donor blood and in the infant at the start and at intervals during the transfusion. Before transfusion the plasma TSH levels of the infants fell as postnatal age indreased and plasma T3 and T4 were correlated with one another. In 20 transfusions the mean infant/donor ratio of TSH was approximately 10, of T4 3, and of T3 2. During these transfusions there was a progressive fall in the infant's plasms TSH, T4, and T3 concentration. In 3 transfusions in which the donor plasma TSH was greater than that of the infant, plasma TSH levels rose during the transfusion and in 2 cases this was associated with a late rise in plasma T3 levels. The addition of glucagon to donor blood had no effect on thyroid hormone levels. It is concluded that erythroblastotic infants have normal thyroid function and that they became biochemically hypothyroid during transfusion. Acute changes in plasma thyroid hormone and glucagon concentration do not induce TSH responses by the neonatal pituitary during the period of the exchange transfusion.