Recruitment of primordial follicles is essential for female fertility. Some of the intraovarian growth factors involved in the initiation of primordial follicle growth have been identified, but the exact mechanisms regulating follicle activation are poorly understood. Strong evidence indicates that vasoactive intestinal peptide (VIP), a neuropeptide found in ovarian nerves, plays a role in the physiology of follicle development and function. The aim of the present study was to determine whether VIP might regulate the activation and growth of neonatal rat primordial follicles in an in vitro culture system. Ovaries from 4-day-old rats were cultured for 14 days in medium containing 10⁻⁷ M VIP. At the end of the culture, the developmental stages and viability of the follicles were evaluated using histological sections. Immunohistochemistry studies for proliferating cell nuclear antigen (PCNA) were performed to assess the mitotic activity of granulosa cells. In addition, the expression level of kit ligand (KL) mRNA was examined after culture. Histology showed that primordial follicles could survive and start to grow in vitro. The proportion of primordial follicles was decreased and the proportion of early primary follicles increased after in vitro culture with VIP. Immunolocalization of PCNA showed that follicle growth was initiated after VIP treatment. The expression level of KL mRNA was increased in the VIP treatment group. Thus, VIP can promote primordial follicle development, possibly mediated in part through upregulating the expression of KL.