The objectives of these experiments were to determine the concentration of corticosterone, the principal adrenocortical steroid in avian species, in serum of turkey embryos (12 to 28 days of development), newly hatched turkey poults (pipped and up to 6 hr posthatch), and poults between 1 and 15 days of age. The effect of giving exogenous doses of corticosterone at a physiological concentration on hatching was also determined. A significant (P less than .01) increase in serum corticosterone from .69 to 1.26 ng/ml occurred in the embryos between 17 and 18 days of development; a second significant increase to 4 ng/ml was just prior to hatching. A decrease in serum corticosterone concentration was noted during hatching and for the first 6 hr after hatching. Concentrations of corticosterone in serum fluctuated as posthatch corticosterone secretion became established between 1 and 15 days posthatch. Treatment of embryos 2 days prior to hatching with exogenous doses of corticosterone (540 ng/embryo) suggested a trend toward shorter mean incubation time and significantly increased hatchability.