During early mouse embryogenesis, the activity of X‐chromosomally linked maternal and paternal phosphoglycerate kinase (PGK‐1) alleles was determined using electrophoretic separation of their gene products and a sensitive fluorometric enzyme assay. In the embryos collected from females homozygous for PGK‐1^b mated with PGK‐1^a males and vice versa, the paternally derived allozyme was first detected after implantation on day 6. Expression of the maternally inherited allele was studied in embryos from females heterozygous for PGK‐1^b and PGK‐1^a. From day 1 to day 4, the embryos maintained a constant ratio of enzyme activity of PGK‐1B to PGK‐1A. Prior to implantation of the embryos between day 4 and day 5, the activity ratio of the two PGK‐1 allelic variants changed significantly due to the first appearance of newly synthesized PGK derived from the maternally inherited allele.

Our data demonstrate a temporal difference in the onset of PGK synthesis depending on whether this particular gene product is of maternal or paternal origin. Therefore, we conclude that the maternal PGK‐1 locus is already activated during late preimplantation development whereas the paternally inherited gene locus remains silent at the preimplantation stage but is subsequently expressed at approximately the time of X‐chromosomal inactivation.