Mammalian embryo culture systems can be used to study dominant lethal, recessive lethal, and visible mutations that are expressed between fertilization and early stages of organogenesis. The incidence of dominant lethal mutations has been determined for cultured mouse embryos by morphological observations during pre-implantation and early post-implantation growth in vitro, and the mechanisms of dominant lethal mutations have been studied cytogenetically. Recessive lethal mutations have been studied with cultured embryos, although they can be initially detected only with appropriate breeding protocols. Visible mutations that are due to deletions or single-site base alterations in the DNA can be detected in embryos by isoelectric focusing and gel-electrophoresis techniques.--The principal advantage of cultured embryos for detecting mutations and studying the mechanism of action of particular mutagens is that embryos are accessible for analysis before death with a minimum of indirect maternal effects. The primary disadvantages, which may be alleviated with improved culture conditions, are that only a limited amount of tissue is available, attrition and retardation occur, and offspring are not recovered for further breeding studies.