Investigations of strains of mice defective in germ cell development have revealed the importance of oocytes for the initial stages of folliculogenesis (Pellas et al., 1991; Huang et al., 1993). Various aspects of follicular development are dependent upon and/or influenced by the presence of oocytes, including granulosa cell proliferation (Vanderhyden et al., 1990, 1992) and cumulus expansion (Buccione et al., 1990; Salustri et al., 1990; Vanderhyden et al., 1990; Vanderhyden, 1993). We are investigating the possibility that oocytes influence one of the primary functions of granulosa cells: steroidogenesis. In many species, granulosa cells removed from preovulatory follicles luteinise in vitro (Channing et al., 1982), presumably due to loss of contact with follicular luteinisation inhibitory factor(s). Indeed, follicular fluid can prevent granulosa cell luteinisation in vitro (Ledwitz-Rigby et al., 1977). Follicular fluid, however, may simply be the medium for transport of factors secreted by oocytes to regulate granulosa cell activities.