Serum samples from male and female zebra finches ranging in age from 1 day before hatch to 54 days posthatch were assayed for 17 beta-estradiol (E), androgen, testosterone (T), or 5 alpha-dihydrotestosterone (DHT). Additional samples were assayed from intact and gonadectomized adults, gonadectomized adults with intraperitoneal implants of testosterone propionate (TP) or estradiol benzoate (EB), gonadectomized nestlings, and nestlings injected subcutaneously with EB. DHT levels of developing birds did not vary as a function of either sex or age. During development, average androgen and T levels were highest during the nestling period, prior to sexual maturation, and were higher in females than in males. Endogenous androgen levels of most subjects that were sampled repeatedly rose and then declined between 24 and 49 days. TP implants produced higher T levels in adult females than in adult males. Levels of E were higher in both sexes during the hatching period (Days -1 through 0) than during the nestling period (Days 2 through 14). A greater number of males than females had relatively high E levels on Days 12 and 14 and during the second week overall. There was no sex difference in levels of E in adults, and gonadectomized adults had markedly higher E levels than intact adults. Gonadectomized nestlings had the same androgen and E levels as intact nestlings of the same age; EB injected nestlings had elevated E levels. The present results indicate specific endocrine changes that mirror events crucial to sexual differentiation of endocrine and behavioral components of reproduction, and have important implications for models of sexual differentiation in zebra finches.