Ring doves (altricial development) and Japanese quail (precocial development) were used as models to compare differences in serum free hormone concentrations and the binding of thyroid hormones to serum protein fractions in adults, and the pattern of free thyroid hormones in the serum of altricial vs. precocial young. Total and free hormones were determined directly by RIA; free hormones also were determined by equilibrium dialysis. Binding protein fractions were identified by electrophoresis of serum preincubated with labelel hormones. Albumin bound the largest proportion of T4 in serum to both species; albumin also bound the largest proportion of T3 in doves, but globulin bound the largest proportion in quail. There were significant differences between species in the proportional binding of both thyroid hormones by different protein fractions at physiological pH. Electrophoretic separations at alkaline pH significantly altered hormone binding by different protein fractions from that at physiological pH. These data explain some conflicting results in the literature on thyroid hormone-binding proteins in different species. Free T4 and free T3 were below the sensitivity limits of the assays during the perinatal period in doves. After hatching, serum free T4 rose more rapidly than total T4. After day 12, hormone concentrations decreased, with a proportionately greater change in free T4 than in total T4. Serum free T3 concentrations were variable, but did not change significantly during development. These results demonstrate that the pattern of serum free thyroid hormones, like that of total hormones, is markedly different in altricial than in precocial development.