1. Specific radioimmunoassays have been employed to measure triiodothyronine (T 3 ), and thyroxine (T 4 ) 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 T 3 was 332±136 (SD) pg/ml. In the paired series the mean cord serum T 3 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.