The functioning of the hypothalamic pituitary-adrenocortical system was tested in young pigeons before and after hatching, by radiocompetitive assay of plasma corticosterone (B) at rest and under stress. 1 degree B was expressed in ng/ml. Resting adult values (8-10) could not be observed before the birds were 6 week-old. B levels were very low 36 h before and 24 and 48 h after hatching (3-5). Then, they progressively increased. 2 degrees B concentrations soared during the 24 h immediately preceding hatching to rise to 45 ng/ml at the time of hatching. After that, corticosteronemia rapidly decreased. 3 degrees Neurogenic stress was applied by means of electrical stimulation of the led skin for 15 sec. Ten to 12 min later, B rose markedly (30 ng/ml) in 6 week-old pigeons as well as in adult ones. No response could be observed from 36 h before to 24 h after hatching. Significant adrenocortical activation was obtained in 3-7 day old squabs (18 ng/ml). 4 degrees Systemic stress (ether vapor) led to significant hypercorticosteronemia (30-40 ng) in pre-hatching embryos. No response could be observed for 24 h after hatching. Ether-induced adrenocortical activation appeared again in 2-3 day-old pigeons. 5 degrees The development of both resting and stress-altered corticosteronemia did not seem to be related to the increase in body weight of young birds. One can present the hypothesis that a strongly stressfull situation occurring during the period which immediately precedes hatching, results in a rapid activation of the adrenocortex. The period following hatching corresponds to the "non stress responsive period" of mammals and could be explained by some feedback mechanisms.
Animals Columbidae