Pituitary adrenocortical response to stress during the first day of post-natal life in the rat.

During the first hours of post-natal life, the newborn rat does not respond to ether stress by a rapid increase of ACTH secretion. Ether inhalation by 5- and 12-hour-old animals slightly but significantly increases plasma and adrenal corticosterone levels, but only 30 min after the onset of stress. At 1 day of age, adrenal response is established. The adrenal cortex of 5-hour-old animals responds normally to an intraperitoneal injection of 25 mU of ACTH. These results indicate that there is an absolute stress non-response period in newborn rats which is followed by a relative stress non-response period. The lack of response is not due to an organic block of the adrenal cortex, but could be due to the accumulation of elevated levels of corticosterone in the central nervous system.