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We have been using an original serum-free chemically defined model system for investigation of developmental processes in gastrulating rat embryo. By this method 9, 5 days old rat embryos without extraembryonic membranes are cultivated on the air-liquid boundary for two weeks in serum-free medium without protein supplements. In such system parameters as survival, growth and differentiation of embryos can be followed by simple methods. 5-azacytidine (5azaC), demethylating agent and teratogen, was applied to the serum- and protein-free culture medium as well as to serum-free medium supplemented with human transferrin. Only one dosage of 5azaC impaired survival, growth and differentiation of embryos in serum-free medium, while in our recent as well as in previous experiments in serum-supplemented medium only repeated application of 5-azacytidine caused changes of development. We might conclude that in simple chemically defined medium it was easier to discover activity of 5-azacytidine than in medium with serum . So, this serum-free in vitro method can probably be used further on to screen for teratogenic or embryotoxic substances at the stage of gastrulation which is the most critical stage of mammalian development.