Isolated oocytes will resume meiosis spontaneously in vitro whereas follicle-enclosed oocytes will remain in the dictyate stage when cultured unless they have been exposed to gonadotropins in vivo or in vitro. To examine the source of the follicular inhibitory influence, porcine oocytes have been cultured alone, with hemisections of follicle wall, granulosa cells, or with follicular fluid. Oocytes isolated from medium-sized (3-5 min) follicles resumed meiosis when cultured; 77.5 plus or minus 3.4 percent matured beyond the dictyate stage. When oocytes were cultured in the presence of follicle wall hemisections of medium and large (6-12 mm) follicles, the percentage of maturing oocytes was significantly reduced. The maturation of oocytes cultured in a medium containing 50 percent follicular fluid from small or large follicles was significantly inhibited. Resumption of meiosis was completely inhibited by co-culture of isolated oocytes with 10-7 granulosa cells from small, medium or large follicles. Addition of serially diminishing amounts of granulosa cells from 10-7 to 10-4 cells reduced the inhibitory influence. It is concluded that the granulosa cells are responsible for the maintenance of the oocytes in the dictyate stage within the follicle. The granulosa cells appear to exert their inhibitory influence upon meiosis by secretion of a chemical message into follicular fluid.