Conceptuses removed from the rats in the eleventh day of gestation were cultured in vitro for 2 days. Growth and differentiation of the major organs of the embryo in vitro resembled those developed in vivo. Embryonic development and organogenesis were markedly affected when the alkylating agents TEM (2,4,6-triethylenimino-1,3,5-triazine) and nitrogen mustard (mechlorethamine hydrochloride) were added to the culture medium. At concentrations of 1 and 5 micrograms/ml medium, these teratogens were highly embryotoxic and affected both growth and differentiation. DNA and protein content of embryos and yolk sacs was reduced significantly (p<0.001) from the controls. Development of conceptuses in the culture medium that contained cyclophosphamide (0.35 mM) alone apparently were normal. However, addition of cyclophosphamide (0.35 mM), microsomes (0.5 mg protein/ml), and NADPH (1 mM) to the culture medium induced marked deleterious effects on the conceptus growth and defferentiation. DNA and protein contents were significantly (p>0.001) reduced by the combined treatment indicative of formation of reactive metabolites and their interaference with macromolecular biosynthesis. Aminopyrine which has not been shown to be teratogenic, in equimolar amounts to cyclophosphamide, under identical conditions of culture, did not affect conceptus development.