ACTH and corticosterone exert opposite effects on the approach and imprinting behavior of newly hatched ducklings. Wild mallard and domesticated Pekin ducklings differ in the early posthatch period in both plasma corticosterone levels and approach/avoidance behaviors. Injection of Pekin duckling embryos with pituitary—adrenocortical hormones alters both later adrenal function and certain aspects of posthatch behavior. These birds have behavioral and hormonal characteristics which resemble those of wild mallards. The hypothesis that behavioral differences in wild and domesticated ducklings result from a higher level of pituitary adrenal function in the wild embryo is explored. Although adrenocortical function changes during domestication in many species, evidence that the hormonal changes mediate the concomitant changes in approach and avoidance behavior remains inconclusive. Factors which cause adrenal function and early behaviors to differ in wild and domesticated genotypes must be sought in the gene action during embryonic development. Since imprinting behavior is modulated by pituitary—adrenal hormones, any factor which affects post—hatch adrenal function may potentially affect imprinting. Later behavior development in the adult is strongly dependent on neonatal experiences; and, therefore, hormonal modulation of early imprinting behavior may constitute an important determinant of adult social behavior.