The toxic effects of the CYREN EC insecticide (480g/l chlorpyriphos) applied alone or in combination with lead acetate were studied on chicken embryos in the early phase of development. The eggs were injected by 0.1 ml of lead acetate solution (0.01%) and/or by 0.1 ml of CYREN EC (0.5%). The treatments were performed on day 0 of incubation. Subsequently, on day 3 of incubation permanent preparations were made from the embryo in order to study the early developmental stage. Embryos fixed on slides and stained with osmium tetroxide solution were studied under light microscope. According to the result of the statistical evaluation, the embryo mortality was significantly higher due to the single or concomitant treatment of CYREN EC. Same tendency was observed in the case of developmental aberration. The single administration of lead acetate increased the embryo mortality and the incidence of developmental anomalies, but the changes were not statistically different as compared to the control. Based on the results, there is a presumably additive type toxic interaction between the lead acetate and CYREN EC that can highly reduce the viability of the embryos or can lead to extinction of wild birds in serious case.