In rodents, parts of the arginine-vasopressin (AVP) neuronal system are sexually dimorphic with males having more AVP-immunoreactive cells/fibers than females. This neuropeptide neuronal system is highly sensitive to steroids and has been proposed to play an important role in the processing of olfactory cues critical to the establishment of a social memory. We demonstrate here that gonadally intact male aromatase knockout (ArKO) mice, which cannot aromatize androgens into estrogens due to a targeted mutation in the aromatase gene, showed severe deficits in social recognition as well as a reduced AVP-immunoreactivity in several brain regions. To determine whether this reduction is due to a lack of organizational or activational effects of estrogens, we assessed social recognition abilities and AVP-immunoreactivity in male ArKO and wild-type (WT) mice when treated with estradiol benzoate (EB) in association with dihydrotestosterone propionate (DHTP) in adulthood. Adult treatment with EB and DHTP restored social recognition abilities in castrated ArKO males since they showed normal female-oriented ultrasonic vocalizations and were able to recognize an unfamiliar female using a habituation-dishabituation paradigm. Furthermore, adult treatment also restored AVP-immunoreactivity in the lateral septum of ArKO males to levels observed in intact WT males. These results suggest that social recognition in adulthood and stimulation of AVP expression in the adult mouse forebrain depend predominantly on the estrogenic metabolite of testosterone. Furthermore, our results are in line with the idea that the organization of the AVP system may depend on androgen or sex chromosomes rather than estrogens.