Recent studies have demonstrated the occurrence of endocannabinoid synthesis and of gene expression and immunoreactivity for the cannabinoid CB1 receptor in the anterior pituitary gland. Since the activity of this gland is under the influence of circulating sex steroids, the present study was designed to elucidate whether expression of the CB1 receptor gene in the anterior pituitary gland is also under the influence of these steroids. To this aim, we first examined the possible changes in the levels of CB1 receptor-mRNA transcripts in the anterior pituitary gland of intact male rats and normal cycling female rats at the different stages of the ovarian cycle. We observed that males had higher levels of CB1 receptor-mRNA transcripts than females. In addition, these transcripts fluctuated in females during the different phases of the ovarian cycle, with the highest values observed on the second day of diestrus and the lowest on estrus. In these animals, we also measured the content of endocannabinoids in the anterior pituitary gland and the hypothalamus. We observed that females had higher contents of anandamide than males in both cases. The content of anandamide in females also fluctuated during the ovarian cycle in both the anterior pituitary gland and the hypothalamus. The highest values in the anterior pituitary gland mere found in the estrus and the lowest on the first day of diestrus and proestrus, whereas the inverse tendency was found in the hypothalamus. No changes were observed in the other major endocannabinoid, 2-arachidonoyl-glycerol, between males and females and during the ovarian cycle. To further explore the potential influence of circulating sex steroids on CB1 receptor gene expression in the anterior pituitary gland, as a second objective, we examined the possible changes in the amount of transcripts for this receptor in gonadectomized and sex steroid-replaced gonadectomized rats of both sexes. We observed that orchidectomy (ORCHX) in males reduced CB1 receptor-mRNA levels, whereas replacement with dihydrotestosterone also maintained low levels of this messenger. In females, estradiol-replaced ovariectomized (OVX) rats exhibited significantly lower CB1 receptor-mRNA levels than OVX animals that had not been replaced with this estrogen. In this experiment, we also examined if the previously reported response of the CB1 receptor gene in the anterior pituitary lobe to chronic administration of Delta(9)-tetrahydrocannabinol (Delta(9)-THC) is under sex steroid influence. We observed that chronic Delta(9)-THC treatment decreased CB1 receptor-mRNA levels in intact and ORCHX males, but not in dihydrotestosterone-replaced ORCHX males. In females, Delta(9)-THC treatment produced no effect in both OVX- and estradiol-replaced OVX rats. In summary, these data collectively support that expression of the CB1 receptor gene in the anterior pituitary gland is regulated by sex steroids in both males and females. Furthermore, gonadal steroids appear to affect the response of this gene to chronic cannabinoid administration. We have also observed that anandamide contents in the anterior pituitary gland and the hypothalamus might be controlled by circulating sex steroids. The functional implications of these data are discussed.