Two-cell mouse embryos were X-irradiated in the late G2 phase in vivo. The first and second postradiation mitoses were analyzed for chromosomal anomalies. The majority of structural aberrations visible at the first mitosis after irradiation were chromatid breaks and chromatid gaps; only a few interchanges and dicentrics were observed. The aberration frequency resulted in a dose-effect relationship which was well described by a linear model. At the second mitosis 29% of the structural aberrations of the first mitosis were counted; the aberration quality changed only slightly. It is discussed whether these aberrations are to be considered "new," "derived," or unchanged transmitted aberrations. Contrary to the results obtained after irradiation of one-cell embryos, little chromosome loss was induced by radiation in two-cell embryos.