On the basis of evidence just reviewed, the hormonal requirements during IVM of mammalian oocytes for fertilization and developmental competence may be summarized as follows: 1. Nuclear maturation occurs "spontaneously," without follicular factors or hormones, in all mammalian species. 2. Cumulus cells are necessary during nuclear maturation in culture for fertilization and developmental competence. Their effect is enhanced by the addition of serum or follicular fluid to the maturation medium. 3. FSH retards nuclear maturation, perhaps thereby enhancing subsequent developmental competence by allowing more time for cytoplasmic maturation to occur before fertilization. The effects of FSH are mediated, in part, by soluble factors from cumulus or granulosa cells. 4. FSH in vivo before culture or added in vitro to IVM culture media enhances fertilization and stimulates steroidogenesis (progesterone and estradiol) in cumulus cells. Blockade of steroidogenesis prevents the enhancing effect of FSH on fertilization. Progesterone can substitute for FSH in enhancing fertilization of rat oocytes. 5. Other hormones enhance beneficial effects of FSH, with some apparent species differences: a. Rat: Progesterone may mediate the FSH effect. b. Cow, rabbit: Estradiol enhances the beneficial effect of FSH. c. Rabbit: Prolactin further enhances the effects of estradiol and FSH. d. Human: No evidence is available on the possible synergism between steroids and gonadotropins in enhancing in vitro maturation. It is clear that further research is required to elucidate the mechanisms of action of all the hormones that have been observed to influence oocyte maturation, and to establish the physiologic significance of most of these, particularly in species other than laboratory rodents. Major benefits to animal production and human health are likely to accrue from such research.