We compared the developmental pattern of thyroid function and its pituitary control in precocial Japanese quail and altricial Ring doves. We measured thyroid hormone (TH) content of thyroid glands (TG), TH concentrations in the serum, relative pituitary gland thyrotropin (PG-TSH) content, and the ability of the TG to respond to exogenous TSH as indicated by increases in TG-3',5'-monophosphate (cAMP) content or in serum TH concentrations. In embryonic quail there is considerable maturation of thyroid function prior to hatching. TG-TH content is low but detectable in Day 8 embryos; TG-TH content increases 300x between Day 8 and hatching (16.5-day incubation). Pituitary TSH is detectable by bioassay in quail on embryonic Day 8, with higher levels found closer to hatching. The TG of 8-day embryos responds to TSH injection by increased TG-cAMP content but the serum TH response to TSH does not appear until Day 9. Serum TH concentrations suggest that the TG is under pituitary control during the latter part of incubation. In doves most of the development of thyroid function and the maturation of its pituitary control occur after hatching and thus thyroid functional development is much later in doves than in quail. TG-TH content is extremely low in embryos and nestlings up to 3 days after hatching, increases slowly in nestlings up to Day 10, then increases sharply. Serum TH content is very low in embryos and rises steadily in nestlings to plateau after about Day 8. Pituitary TSH content, estimated by a quail bioassay, is undetectable in embryos and nestlings until Day 4 but increases thereafter. The TG does not respond (based on serum TH concentration) to TSH injection through the day of hatching (Day 16; mean incubation period of 16.5 days), but an increase in serum TH occurs in Day 2 nestlings in response to TSH injection. The magnitude of this response continues to increase during the first week after hatching.