Plasma levels of both adrenocorticotropic hormone (ACTH) and corticosterone (B) were determined in embryos (day 15 of incubation), chicks (day 3 after hatch) and young chickens (8 weeks). Experimental animals were bursectomized at 80 hr of incubation, i.e., before any anlage of the bursa of Fabricius could develop. Bursectomized (BFX) animals were compared to sham-operated controls (T), in basal, resting condition and 7 (ACTH) or 14 min (B) after ether stress was delivered for 30 sec. Basal B and ACTH levels seemed not to be significantly modified in BFX embryos, chicks and chickens. Hypophysial and adrenocortical response to stress appeared more precociously in BFX embryos (day 15 of incubation) than in intact ones (day 19). The non stress-responsive period that was observed for one week after hatch of T birds did not appear in 3-day-old BFX chicks whose both B and ACTH stress-induced levels were as high as in intact adults. In contrast, adrenocortical and pituitary corticotropic responses to stress were markedly impaired (by 50%) in adult BFX chickens as compared to intact controls.