[en] In the experiments, chick embryos between the primitive streak stage and the 6-somites stage were used. These embryos were cultured in vitro, with the presence of Parathion for 12, 24, 36 hours. The result showed that Parathion has a teratogenic effect on the early stages of embryogenesis. Morphological and histological changes in brain, embryonic axis, heart mesenchyme and erythrocytes were found. Parathion's effect at molecular level quantitatively and qualitatively was studied, using autoradiography and radioactivity measurements after incorporation of ³H thymidine, ³H uridine and ³H delta amino levulinic acid. An inhibitory effect of Parathion on the synthesis of DNA, RNA and hemoglobin was shown. The inhibitory ratio increases with the increase of exposure period to Parathion. These results suggested that Parathion affects other mechanisms which have no relation with acetylcholinesterase activity. This activity was inhibited in a more advanced stages by Parathion. (author)