The effects of activin A and/or follistatin on the development of bovine embryos were investigated. Presumptive zygotes matured and fertilized in vitro were cultured in a chemically defined medium (modified synthetic oviduct fluid medium; mSOF). Addition of 1–100 ng/ml of activin A to mSOF significantly increased the percentage of zygotes that developed to morulae and blastocysts (48–54% and 31–41%, respectively) compared with no addition (41% and 25%, respectively). In contrast, addition of 1–100 ng/ml follistatin significantly reduced the percentage of zygotes developing to morulae and blastocysts (29–31% and 17–20%, respectively) compared with no addition (41% and 28%, respectively). In a culture with 10 ng/ml of activin A, supplementation with the same concentration of follistatin neutralized the positive effect of activin A, while supplementation with 100 ng/ml of follistatin reduced the percentage of zygotes that developed. The total cell numbers in morulae and blastocysts were not affected by the addition of activin A and/or follistatin. The development-enhancing effects of activin A and the development-impeding effects of follistatin were observed when embryos were exposed to activin A or follistatin at a concentration of 10 ng/ml prior to the 9- to 16-cell stage. These results suggest that activin A and follistatin may affect bovine embryos until the third cell cycle and may play important roles in regulation of the developmental competence of bovine embryos.