The present work studies the possibility of inducing an alteration in the erythropoietic switchover from primitive to definitive red cells in the chicken embryo by means of Cytoarabine (Ara C). To this purpose two different concentrations of Ara C were injected into fertilized chicken eggs after 94 hours of incubation. Optical microscopy was used in the morphological analysis of the blood smears taken from treated and control embryos on the 7th and 8th day of incubation (state 30 and 33 in the Lillie Table). 1) Injection of Ara C 4.1 x 10(-5) M did not induce any significant differences in the hematological picture between treated embryos and the controls. 2) Injection of Ara C 4.1 x 10(-4) M revealed a delay in the erythropoietic switchover both at the 7th and 8th day of incubation. In fact, the peripheral blood of the treated embryos showed that in the treated embryos there was a more than 24 hour delay in erythropoietic maturation. This is indicated by an unusually high number of 1st generation erythrocytes in the peripheral blood. Moreover, the 2nd generation red cells are at a lower stage of cell differentiation than the controls.