Sexual dimorphism of the rodent brain is manifested by the epigenetic action of gonadal steroids. Our previous research identified the granulin (grn) precursor gene as a sex steroid-inducible gene, which was shown to be expressed more abundantly in male than female neonates at the mediobasal hypothalamic area. Grn is a 6-kDa polypeptide promoting or inhibiting the growth of epithelial cells and hematocytes in vitro. In this study, effects on male sexual behavior of male were pursued under conditions in which grn gene expression was suppressed during the critical period. To this end, an antisense oligodeoxynucleotide (ODN) of the grn precursor gene was designed, incorporated into inactivated Sendai virus (HVJ)-liposome complexes, and infused into the third ventricle of 2-day-old male rats. Two different control treatments were used: the first consisted of a control sequence ODN that had little homology to known mRNAs; the second of vehicle (HVJ-liposome) alone. After maturation, animals treated with antisense ODN of grn displayed significantly lower scores than control males on various parameters assessing sexual behavior; i.e., mount, intromission, and ejaculation. The antisense ODN, however, did not affect body growth or serum concentrations of testosterone and luteinizing hormone. Further, there was no significant difference in the volume of the sexual dimorphic nucleus of the preoptic area between antisense ODN-treated and control animals. It was shown that inadequate expression of the grn gene in the brain of male neonatal rats during the critical period suppressed the induction of some type of male sexual behavior, suggesting the grn was involved in the process of masculinization of the rat brain.