A whole follicle culture system has been used to investigate the actions of gonadotrophic hormones, oestrogen and progesterone in the regulation of follicular development and steroidogenesis. Recombinant human FSH was required for the growth of preantral follicles and for Graafian morphogenesis, whereas recombinant LH was ineffective. While pure FSH was sufficient for growth and morphogenesis, production of oestrogen was greater when androstenedione or LH was present in combination with FSH, confirming that there is a two-cell mechanism for oestradiol production in the mouse follicle. When an antiserum to oestrogen or to progesterone or an oestrogen receptor antagonist were added to the culture medium, there was no significant effect on either follicular growth or oestradiol production. Thus, physiological concentrations of oestradiol are not needed for follicle development, although a role cannot be completely ruled out. In conclusion, the obligatory role of FSH was demonstrated. It appears to be sufficient for follicle development even in the absence of LH, and the paracrine or autocrine effects of oestradiol and progesterone, if any, appear to be minor in the mouse ovary.