The preovulatory surge of gonadotropins induces within the mature Graafian follicle a series of changes culminating in the release of a fertilizable ovum. These include resumption of the meiotic division, a process held in abeyance from a short time after birth, and the progression of the oocyte from the dictyate stage to the metaphase of the second meiotic division. Here the role of a follicular factor, oocyte maturation inhibitor (OMI), in preventing resumption of meiosis by ova of antral follicles prior to the surge of gonadotropins has been reviewed. The suggested involvement of OMI in regulation of meiosis is based on the following observations: (1) fully grown mammalian oocytes explanted from their follicles undergo meiotic maturation spontaneously, whereas follicle-enclosed ova remain immature until stimulated; (2) co-culture of oocytes isolated from their follicles with follicular granulosa cells, granulosa cell extract and follicular fluid inhibits the spontaneous maturation; (3) the inhibition of oocyte maturation by OMI is reversible and in several of the models employed can be removed by the addition of the physiological trigger of meiosis, luteinizing hormone (LH). The current state of OMI characterization and purification has been described and the involvement of additional factors, such as cyclic AMP, in the regulation of meiosis discussed.