It has been demonstrated that in zygotes of some mammals the unique checkpoint controls the onset of DNA replication.Thus, DNA replication begins in the maternal pronucleus only after the paternal pronucleus is fully formed. In our experiments we have investigated whether this checkpoint also operates in porcine zygotes produced either by in vitro fertilization (IVF) or by intracytoplasmic sperm injection (ICSI). Our results show that the onset of DNA replication occurs in the maternal pronucleus even in the presence of an intact sperm head/s in zygotes produced by IVF or ICSI. We conclude that in porcine zygotes there is an absence of DNA replication checkpoint.