BACKGROUND. Arginine vasopressin (AVP) is a nonapeptide expressed in several brain regions. In addition to its well-characterized role in osmoregulation, AVP regulates paternal behavior, aggression, circadian rhythms, and the stress response. In the bed nucleus of the stria terminalis (BST), AVP gene expression is tightly regulated by gonadal steroid hormones. However the degree by which AVP is regulated by gonadal steroid hormones in the suprachiasmatic nucleus (SCN) and medial amygdala (MeA) is unclear. Previous studies have shown that AVP expression in the brain of gonadectomized rats is restored with testosterone, 17-estradiol, and 5-dihydrotestosterone (DHT) replacement. In addition, we have demonstrated that 3-Diol, a metabolite of DHT, increased AVP promoter activity in a neuronal cell line and that the effects of 3-Diol on AVP promoter activity were mediated by estrogen receptor-. METHODS. To test whether 3-Diol has a physiological role in the regulation of central AVP expression in vivo, we gonadectomized pre- and post-pubertal male rats followed by once daily injections of estradiol benzoate (EB), DHT-propionate, 3-Diol-dipropionate, or vehicle. RESULTS. The SCN, BST, and MeA were analyzed for AVP mRNA expression using in situ hybridization. In the BST, intact juveniles had significantly fewer AVP expressing cells than adults. GDX abolished all AVP mRNA expression in the BST in both age groups, whereas treatment with EB restored >80% and DHTP <10% of the AVP expression. CONCLUSIONS. Interestingly, 3-Diol-P was more effective at inducing AVP expression in juveniles than adults suggesting that the regulation of AVP by 3-Diol might be age-dependent.