The neurosteroid allopregnanolone (AP) is a GABAergic agonist that suppresses central nervous system (CNS) activity in the adult brain, and by reducing excitotoxicity is considered to be neuroprotective. A role for neurosteroids in the developing brain, particularly in late gestation, is still debated. The aim of this study was to investigate effects on proliferation and cell death in the brain of late gestation fetal sheep after inhibition of AP synthesis using finasteride, a 5alpha-reductase type 2 (5alpha-R2) inhibitor. Catheters were implanted in fetal sheep at approximately 125 days of gestation. At 3-4 days postsurgery, fetuses received infusions of either finasteride (20 mg/kg/h; n=5), the AP analogue alfaxalone (5 mg/kg/h; n=5), or finasteride and alfaxalone together (n=5). Brains were obtained at 24 h after infusion to determine cell death (apoptotic or necrotic) and cell proliferation in the hippocampus and cerebellum, areas known to be susceptible to excitotoxic damage. Finasteride treatment significantly increased apoptosis (activated caspase-3 expression) in hippocampal CA3 and CA1, and cerebellar molecular and granular layers, an effect abolished by co-infusion of alfaxalone and finasteride. Double-label immunohistochemistry showed that both neurons and astrocytes were caspase-3 positive. Finasteride treatment also increased the number of dead (pyknotic) cells in the hippocampus and cerebellum (Purkinje cells), but not when finasteride+alfaxalone was infused. Cell proliferation (Ki-67-immunoreactivity) increased after finasteride treatment; double-labeling showed the majority of Ki-67-positive cells were astrocytes. Thus, steroids such as AP appear to influence the constitutive rate of apoptosis and proliferation in the hippocampus and cerebellum of the fetal brain, and suggest an important role for neurosteroids in the development of the brain.