Maturation of the oocyte can be divided into two different aspects: nuclear maturation and cytoplasmic maturation. The spontaneous nature of nuclear maturation in oocytes removed from the follicle and cultured in vitro was observed in mammals as early as 1935. However, oocytes cultured in basic conditions are deficient in some cytoplasmic factors and are, therefore, developmentally incompetent. Data from large domestic species indicate that although oocytes matured in vitro in supplemented media can develop after fertilization, they require the presence of follicular factors during culture to ensure their developmental competence. The importance of follicular maturation on the capacity of oocytes to achieve fertilization and early embryonic development can be studied by reproducing some important events in vitro. Follicular supplementation may be in the form of follicular fluid, granulosa cells or follicle-conditioned media, and there is evidence that the maturational status of the follicle used for co-culture influences subsequent male pronuclear formation. Studies in this laboratory on the prolific Chinese Meishan pig, which has significantly higher early embryonic survival than conventional European breeds, indicate crucial differences in the pattern of follicle development and, therefore, the intrafollicular environment in which the oocytes are nurtured. It is suggested that this produces oocytes of improved 'quality' and this hypothesis is supported by experiments both in vivo and in vitro. Ultimately, it is hoped that these studies on large domestic animals will lead to identification of the follicular factors that influence oocyte quality.