Certain reproductive techniques culture embryos in vitro; however, little is known about the impact of culture on fetal growth. Coculture of day 1 ovine zygotes on a bovine granulosa cell layer to blastocysts followed by transfer to synchronous recipients increased fetal weight by 11 and 40% at days 61 and 125, respectively, compared with the transfer of in vivo-produced blastocysts. Plantaris muscle weights were increased by 40% in cultured fetuses at day 125. Examination of myogenesis in plantaris muscle showed that primary fiber number was unchanged at day 61 by culture but that primary fiber area was increased significantly by 15 and 25% at days 61 and 125, respectively; secondary fiber area was increased by 40% at day 125 by culture, and the ratio of secondary to primary fiber numbers was 18-20% greater in the cultured groups compared with the controls at days 61 and 125. The results show that coculture of preimplantation embryos may alter myogenic programming. These changes may contribute to the abnormally large muscles observed in oversize fetuses.