1. Specific radioimmunoassays have been employed to measure triiodothyronine (T3), and thyroxine (T4) concentrations in the cord serum of fifty-five healthy babies and in a second series of fifteen paired cord and maternal blood samples.
2. The mean cord serum T3 was 332±136 (SD) pg/ml. In the paired series the mean cord serum T3 was 355±142 pg/ml whereas the mean maternal value was 1557±283 pg/ml.
3. Serum thyrotrophin (TSH) concentration was raised in cord serum compared with maternal serum.