Studies of follicular morphology in biopsies of human ovarian tissue obtained from women with a syndrome of hypogonadotropic hypogonadism have been reviewed. Follicular maturation was consistent with that seen in ovarian tissues removed at postmortem examination of infants. Similarly, follicular maturation was inhibited in ovaries of women receiving oral contraceptive preparations consisting of a mixture of an estrogen and a progestogen. Considered together, ovarian morphology in these two states of relative gonadotropin deficiency suggests that initiation and progression of follicular maturation require adequate quantities of gonadotropins. The stimulatory effects of gonadotropin on preantral stages of follicular maturation are mediated by estrogen in rat ovaries. Failure to observe follicular maturation in ovaries of women with a congenital inability to synthesize estrogen in response to stimulation by gonadotropin suggests that the steroid may play a similar role in initiating follicular maturation in women.