Seven adult male and 12 newborn female hybrids between M. arvalis (dams) and M. levis (sires) were examined, as well as three adult male M. arvalis, four adult male and three newborn female M. levis. Captive-bred colonies of the parental species were established from individuals trapped in Leningrad district (M. arvalis) and Novosibirsk district (M. levis) and maintained in the animal house of the Institute of Cytology and Genetics. Maintenance, handling and euthanasia of animals followed protocols approved by the Animal Care and Use Committee of the Institute of Cytology and Genetics. Experiments described in this manuscript were carried out in accordance with the approved national guidelines for the care and use of laboratory animals.