effect of acth and histamine stress on serum corticosterone and adrenal cyclic amp levels in immature rats adrenal camp and plasma corticosterone levels were determined in preweanling rats subjected to treatment with either acth 50 murat or histamine dihydrochloride 02 mgg body wt acth injection elevated both serum corticosterone and adrenal camp levels on all days tested however the acthinduced elevation of adrenal camp and serum corticosterone both diminished steadily from day 2 to day 8 and then increased from day 8 to day 16 histamine injection resulted in elevated serum corticosterone levels in a pattern similar to that of the corticosterone response to acth however histamine injection did not result in any significant increase in adrenal camp from day 2 to day 10 from day 12 to day 16 the adrenal camp concentration rose steadily in parallel with ther serum corticosterone levels these results indicate 1 that a functional acthsensitive adenyl cyclase system is present in the adrenal gland of the immature rat 2 that the responsiveness of this system diminishes during the first postnatal week before returning to its previous 2dayold capacity by day 16 and 3 that during the first few days after birth histamine stress results in elevated serum corticosterone levels without elevating adrenal camp levels