Summery The frequency of (dominant and) recessive lethal mutations induced by 160-rad chronic gamma-irradiation given with a dose rate of 0.03 rad/min during the 10th and 14th day of gestation has been studied in female CBA-mice. An increased rate of recessive lethal equivalents by about 10% has been noted. This increase corresponds to a mutation rate of 6.3*10**-4 mutation/rad/genome. There were not found any dominant mutations, nor any dominance effects from the induced recessive lethal equivalents. The hazards after irradiation during foetal development are discussed.