The sexually dimorphic extrahypothalamic arginine-vasopressin (AVP) projections from the bed nucleus of the stria terminalis to the lateral septum (LS) and lateral habenula (LHb) are denser in males than females and, in rats, require males' perinatal exposure to gonadal hormones but the absence of such exposure in females. We examined perinatal hormone effects on development of this sex difference in prairie voles (Microtus ochrogaster), which show atypical effects of hormones on sexual differentiation of some reproductive behaviors. Neonatal castration reduced the number of AVP mRNA-expressing cells in the bed nucleus of the stria terminalis and AVP immunoreactivity (ir) in the LS and LHb. Surprisingly, daily injections of 1000 microg of testosterone propionate (TP) during the first postnatal week did not maintain high levels of AVP-ir in neonatally castrated males. Furthermore, perinatal treatments with TP (75, 500, or 1000 microg), testosterone (100 microg), or dihydrotestosterone (200 microg) did not masculinize AVP-ir in the female LS or LHb. In fact, 1000 microg TP reduced it in some cases. However, 1000 microg TP lengthened anogenital distance, indicating that TP was biologically active. Neonatal estrogen receptor antagonism with tamoxifen reduced AVP-ir in the male LS, whereas treating neonatal females with the synthetic estrogen diethylstilbestrol increased septal AVP-ir. Tamoxifen and diethylstilbestrol had no effects in the LHb. Similar to rats, therefore, postnatal estrogen influences some components of the extrahypothalamic AVP system in prairie voles, but this developing system appears to be insensitive to exogenous androgens, including aromatizable androgens. Such insensitivity is atypical for a sexually dimorphic neural system in a rodent and may reflect the unusual effects of hormones on sexual differentiation of some behaviors in prairie voles.