Astrocytes in the posterodorsal portion of the medial amygdala (MePD) are sexually dimorphic in adult rats: males have more astrocytes in the right MePD and more elaborate processes in the left MePD than do females. Functional androgen receptors (ARs) are required for masculinization of MePD astrocytes, as these measures are demasculinized in adult males carrying the testicular feminization mutation (Tfm) of the AR gene, which renders AR dysfunctional. We now report that the number of astrocytes is already sexually dimorphic in the right MePD of juvenile 25-day-old (P25) rats. Because Tfm males have as many astrocytes as wild-type males at this age, this prepubertal sexual dimorphism is independent of ARs. After P25, astrocyte number increases in the MePD of all groups, but activation of ARs augments this increase in the right MePD, where more astrocytes are added in males than in Tfm males. Consequently, by adulthood, females and Tfm males have equivalent numbers of astrocytes in the right MePD. Sexual dimorphism in astrocyte arbor complexity in the left MePD arises after P25, and is entirely AR-dependent. Thus, masculinization of MePD astrocytes is a result of both AR-independent processes before the juvenile period and AR-dependent processes afterward. J. Comp. Neurol. 521:2298–2309, 2013. © 2012 Wiley Periodicals, Inc.