The role played by nitric oxide (NO) and carbon monoxide (CO) was explored in the adult male rat by determining whether antagonizing the activity of the enzymes responsible for the formation of these gases altered the response of the hypothalamic-pituitary-adrenal (HPA) axis to immune (cytokines) or nonimmune (mild electroshocks) signals. The arginine derivative N<sub>ω</sub>nitro- L-arginine-methylester ( L-NAME), which inhibits all three NO synthase (NOS) isoforms [inducible (i), endothelial (e) and neuronal (n)] significantly augments the ACTH response to blood-borne cytokines, but decreases it in rats exposed to shocks or other physico-emotional stresses. The effect of L-NAME in both models is mimicked by L-nitroarginine ( L-NNA) and L-nitromethylarginine ( L-NMMA), which block constitutive (e and n) forms of NOS, but not by aminoguanidine (which blocks iNOS) or 7-nitroindazole (which specifically blocks nNOS). Despite the ability of L-NAME to markedly augment the stimulatory effect of vasopressin on ACTH secretion, removal of this peptide does not interfere with the interaction between L-NAME and systemically administered interleukin-1β (IL-1β). In contrast, blockade of prostaglandin formation prevents both the stimulatory effect of IL-1β on ACTH release, and its potentiation by L-NAME. In contrast to the investigation of the importance of endogenous NO, studies focused on the role of CO remain scarce. Our preliminary results suggest that while blockade of the formation of this gas decreases the ACTH response to various stimuli, it also significantly interferes with the effect of L-NAME in rats systemically administered cytokines, and further decreases the ACTH response to shocks in animals also injected with arginine analogs. These results indicate the possible presence of functional interactions between NO and CO in regulating the activity of the HPA axis. Our present working hypothesis is that in the presence of elevated circulating cytokine levels, endogenous NO acts presynaptically to inhibit the release of ACTH secretagogues from nerve terminals in the infundibulum. As the acute ACTH response to these immune proteins is believed to primarily depend on events taking place within the median eminence, blockade of NO formation results in exaggerated ACTH release. During exposure to shocks and other nonimmune stresses, on the other hand, increased ACTH secretion is primarily due to activation of hypothalamic neurons. In this case, because of the stimulatory influence of endogenous NO on hypothalamic perikarya that manufacture corticotropin-releasing factor (CRF) and/or of the afferents to these neurons, blockade of NOS activity blunts CRF production, and consequently ACTH release. What remains undetermined is the net effect of the opposite influences of NO during long-term exposure to immune or nonimmune stress. Finally, it is possible that the conflicting results reported by investigators who study the role of NO and CO in isolated cell preparations may reflect, at least in part, these opposite effects of NO on different elements of the HPA axis.