Summary. Twice daily injections of purified ovine and human FSH were used to investigate the control of ovarian follicle development in hypogonadotrophic hypogonadal (hpg) mice. Treatment for 5 days with doses greater than 3 pg resulted in a significant increase in the total number of growing follicles and the development of antral follicles. This was associated with increases in uterine weights and vaginal opening, indicating that steroidogenesis had also been stimulated. Further studies of the effects of combined injections of FSH and LH, linked with morphological analysis of ovarian interstitial cells, suggested that any contribution of background or contaminating LH to the effects of the FSH injections was minimal. It therefore appears that, in mice, FSH alone is capable of stimulating an increase in the initiation of follicle growth, of triggering the development of antral follicles, and supporting ovarian steroidogenesis.