Medial basal hypothalamic (MBH) 5 alpha-reductase activity was significantly blocked with a known inhibitor, Proscar (Finasteride), in pregnant rats while their open-field behavior was quantified during the last week of pregnancy. In control animals, open-field behavior significantly decreased (in a stair-step fashion) as a function of increasing gestational age. Conversely, in Proscar-treated animals open-field values significantly increased on day 15 and 17 of gestation compared to control values. These data indicate that inhibition of MBH 5 alpha-reductase during pregnancy significantly increased open-field activity levels during late gestation in rats and provides evidence for a link between the production of 5 alpha-reduced metabolites of progesterone in brain and behavioral activity during pregnancy.