SUMMARY: Hormonal iodine metabolism was studied in newborn lambs born to sheep which had been injected with 131I 3–11 days earlier. A rapid outflow of the pre-formed foetal hormone occurred immediately after birth causing a loss of 41% of the pre-formed hormone within 15 min and a loss of 60% of iodine pre-accumulated in thyroglobulin 5 h later (as revealed by histochemistry). Postnatal hypersecretion was followed by a short-lived rise in serum protein-bound iodine and free thyroxine (measured indirectly). Assuming that thyrotrophin (TSH) is a major stimulator of the thyroid secretory activity in the newborn lamb, the sequence of events indicates that the enhanced TSH and thyroxine release begins as a result of the stress of parturition rather than as a result of body-cooling thereafter.