The effect of adrenal cortical hormones on basal and prostaglandin-E2 (PGE2)-stimulated levels of cAMP was studied in incubated rat anterior pituitaries. It was found that oral administration of dexamethasone (DEX) at the concentration of 10 micrograms/mL of drinking saline to adrenalectomized (ADX) rats for four days decreases basal and PGE2-stimulated cAMP levels in the pituitary but not in the hypothalamus. Furthermore, a time-course study demonstrated that acute intraperitoneally (IP) DEX treatment had no effect; whereas, after one day of oral DEX, PGE2-stimulated levels, but not basal, were already reduced. The effect of DEX was also obtained with two natural rat adrenal hormones like corticosterone and deoxycorticosterone, whereas, progesterone and testosterone, which share with the corticoids a ring A-unsaturated-three-ketone structure but are devoid of corticoid activity, did not modify at physiological doses the levels of cAMP. More physiological validation of these results was given by experimental procedures that modified endogenous adrenal secretion. Thus, reduction of plasma corticoids by ADX significantly increased basal and PGE2-stimulated cAMP levels in the pituitary, whereas, increments of plasma corticosterone produced by ACTH had the reverse effect in intact rats. It is suggested that adrenal gland regulation of cAMP may reflect involvement of the nucleotide in the mechanism of action of corticoids at the pituitary level.