The purpose of this project is to study the underlying causes of induced embryonic mortality in the dominant lethal test in the mouse. This is achieved by direct observation of the in vitro development of embryos and by cytogenetic analysis of their karyotypes. One of the most widely used tests of mutagenicity in mammals is the dominant lethal assay. While this system allows detection of induced increases in foetal mortality, the underlying causes are not clearly delineated. One of the proposed factors has been the induction of chromosomal aberrations. It is our purpose to investigate the processes and causes of foetal mortality by direct observation of the development of embryos and by cytogenetic analysis of their karyotypes.