In the neonatal rat, the response of the hypothalamo-pituitary-adrenal axis to stressful stimuli is markedly decreased during the first 2 weeks of life. This peculiar period was named "stress hyporesponsive period." In this report, we studied the effect of insulin-induced hypoglycemia, known as a strong stimulator of the corticotroph function in the adult rat. Rats (8- or 20-day-old) were injected ip with 3 IU/kg synthetic insulin and were killed at various times. In 20-day-old rats, hypoglycemia induced a rapid drop in blood glucose concentrations accompanied by a stimulation of ACTH and corticosterone secretion which reached maximal values within 30 min. On the opposite, in 8-day-old rats, despite a rapid decrease in blood glucose levels, insulin injection induced a gradual rise of plasma ACTH and corticosterone concentrations which peaked at 90 min. This delayed response of the hypothalamo-pituitary-adrenal axis to hypoglycemia in the youngest rats does not seem to be due to a difference of sensitivity to insulin-induced hypoglycemia since injection of increasing doses of insulin (0.3, 0.75, or 3 IU/kg body wt) induced a dose-related decrease of blood glucose concentrations and a rise in plasma ACTH and corticosterone levels, comparable in the two age group studied. Basal or hypoglycemia-stimulated absolute corticosterone values were much lower in 8-day-old rats than in 20-day-old animals, suggesting an immaturity of the adrenal glands in the youngest animals. Daily ACTH injection, starting 3 days before the experiment, had a trophic effect on the adrenal glands leading to a more important increase of corticosterone levels after hypoglycemia in 8-day-old rats. Our results confirm that there is an immaturity of the adrenal glands in young rats, probably due to the low plasma ACTH levels during the neonatal period. To determine the respective role of the two major hypothalamic ACTH secretagogues, we studied the effect of passive immunization against CRF or arginine vasopressin (AVP) on plasma ACTH response after hypoglycemia. Passive immunization against AVP decreased significantly hypoglycemia-stimulated ACTH secretion in both 8- and 20-day-old rats, while no change of plasma ACTH response to insulin injection was observed after passive immunization against CRF. This results suggest that CRF does not seem to be involved in the regulation of ACTH secretion after hypoglycemia in the young rat while AVP seems to be the main hypothalamic stimulatory factor for anterior pituitary corticotrophs response to hypoglycemia during the postnatal period.(ABSTRACT TRUNCATED AT 400 WORDS)