Abstract
Plasma corticosterone concentrations in newly hatched ducklings exposed to an imprinting model are inversely related to the strength of approach behavior. Injections of corticosterone before imprinting reduces following, whereas alpha1-10-adrenocorticotropin or antiserum to corticosterone augments following behavior. The sensitive period for imprinting may be regulated by changes in the pituitary-adrenocortical axis.