The thymus gland, besides its well-recognized role in the immune system, plays a role in reproductive biology, particularly in regulating ovarian function. Studies using neonatally thymectomized mice or congenitally athymic nude mice (nu/nu) have clearly shown that thymus gland influences ovarian function, although the mechanism for this influence is not completely known [1], In this chapter, we shall summarize the experimental evidence that supports the regulation of the ovaries by the thymus gland and the mechanisms that might be involved. We further describe our characterization and partial purification of two potentially novel thymic factors that directly influence ovarian steroidogenesis in vitro.