Two experiments were carried out to evaluate the effects of EGF supplementation to maturation medium on the fertilization and developmental potential of bovine oocytes after in vitro fertilization, and to see whether the addition of wortmannin from 6 h after the onset of maturation culture would modulate the action of EGF. The effects of EGF were compared to those of gonadotropins + serum treatment. The presence of EGF in maturation medium significantly increased the number of normally fertilized oocytes. No difference in the proportion of normally fertilized oocytes was observed between EGF and gonadotropins + serum treatments. Wortmannin completely abolished the stimulatory effects of both treatments on fertilization, and effectively increased the number of unfertilized oocytes. The rate of cleavage, and the proportions of embryos reaching the blastocyst and hatched blastocyst stages after in vitro fertilization were also significantly improved with the addition of EGF to maturation medium. The presence of gonadotropins + serum during in vitro maturation resulted in rates of development statistically similar to that obtained with EGF alone. Wortmannin effectively abolished the stimulatory effects of either treatment on cleavage and embryonic development. These data suggest some physiological role for EGF on cytoplasmic maturation, and indicate the importance of PI 3-kinase activity in the process of cytoplasmic maturation.