The purpose of this study was to investigate the effects of the endogenous cannabinoid arachidonoyl-ethanolamide, anandamide (AEA), on the activity of the hypothalamo-pituitary-adrenal (HPA) axis in cannabinoid receptor (CB<sub>1</sub> receptor) inactivated (KO) mice. A low dose (0.01 mg/kg i.p.) of AEA significantly increased plasma corticotropin (ACTH) and corticosterone concentrations in both wild-type (+/+) and in mutant (–/–) animals. In each case, hormone levels reached their peaks at 90 min after AEA administration. In a parallel experiment, AEA administration was preceded by the injection of SR 141716A (1.0 mg/kg), a selective and potent CB<sub>1</sub> receptor antagonist, or of capsazepine (5.0 mg/kg), a potent vanilloid receptor of type 1 (VR1) antagonist. The latter drugs did not prevent the effects of AEA on the HPA axis. Using Fos protein immunohistochemistry, we observed that the parvocellular part of the hypothalamic paraventricular nucleus (PVN) was activated as early as 45 min after AEA injection and reached peak levels after 60 min in both +/+ and –/– mice. Furthermore, the CB<sub>1</sub> and VR1 receptor antagonists did not block the effects of AEA on Fos immunoreactivity. The results strongly support the view that activation of the HPA axis produced by AEA possibly occurs via a currently unknown (CB<sub>x</sub>) cannabinoid receptor present in PVN.