A quantitative electrophoretic analysis of glucose phosphate isomerase (GPI-1) allozymes produced by heterozygous Gpi-1sa/Gpi-1sb mouse embryos has enabled us to estimate separately the contributions of GPI-1 enzyme that were oocyte coded, encoded by the embryonic, maternally derived Gpi-1sa allele and encoded by the embryonic, paternally derived Gpi-1sb allele. The oocyte-coded GPI-1 activity is stable until 2 1/2 days and then declines and is exhausted by 5 1/2 to 6 1/2 days post coitum (p.c.). The maternally and paternally derived Gpi-1s alleles are probably usually activated synchronously but several possible exceptions were observed. This activation was first detected in 2 1/2-day embryos. Total GPI-1 activity falls to a minimum around 3 1/2 to 4 1/2 days, even though embryonic gene expression has already begun. The profile of oocyte-coded GPI-1 activity is consistent with the suggestion (Harper & Monk, 1983) that there is a mechanism for the removal of oocyte-coded gene products at around 2 1/2 days p.c. The method of analysis described is applicable to other dimeric enzymes with electrophoretic variants.