Adult male mice were injected intraperitoneally with triethylenemelamine (TEM) at 0.3 mg/kg. These males were mated with untreated females twice a week during 24 days after the treatment. On day 3 of gestation, embryos were flushed out from uteri and examined cytologically as well as cytogenetically. A dominant-lethal test was conducted using the male treated in the same way. Paternal treatment of TEM caused developmental retardation of the embryos obtained from the matings on 20 post-injection days. These embryos frequently showed micronuclei in interphase cells and structural chromosome aberrations in methaphases. Most of these aberrations were chromosome types, such as breaks and exchanges, and premature chromosome condensations. The developmental retardation as well as the frequency of chromosome aberrations was most marked in the matings on post-injection day 13. Similarly, the highest dominant-lethal effect was shown in the group mated on post-injection days 11--13. It was concluded that TEM induces chromosome damage in the post-meiotic germ cells and then this damage in turn produces chromosome aberrations in the embryos, resulting in high incidence of dominant lethality.