Exogenous thymidine affects morphogenesis of the early chick blastoderm possibly by depleting the deoxycytidine triphosphate pool. The aim of this study is to determine whether the inhibitory action of thymidine on early chick blastoderm morphogenesis is alleviated by the removal of thymidine and/or treatment with deoxycytidine. Chick blastoderms at the full hypoblast stage develop abnormally in egg albumen containing 1.23 X 10(-3) M thymidine. Development is normal when deoxycytidine is included simultaneously in the culture medium with thymidine at equimolar concentrations. Blastoderms were cultured in egg albumen containing 15 microCi/ml thymidine [methyl-3H] or 10 microCi/ml deoxycytidine [5-3H], and 1.2 X 10(-3) M 2'-deoxycytidine or 1.23 X 10(-3) M thymidine, respectively. The culture was interrupted at timed intervals, and the amount of radioactivity associated with DNA was determined. Exogenous deoxycytidine in the culture medium caused a noticeable increase in the incorporation of 3H-thymidine, while exogenous thymidine markedly inhibited the uptake and incorporation of 3H-deoxycytidine into DNA of blastoderms. Thymidine does not inhibit the expansion of blastoderm, the migration of cells for formation of the primitive streak (PS), and the induction of axial tissues, but it interferes with the organization of these tissues to form the embryonic axis. Blastoderms show slight signs of recovery when thymidine is removed. Deoxycytidine counteracts the action of thymidine and seems to be a rate-limiting factor in normal differentiation of the early chick blastoderm.