The early embryonic events involved in the commitment of mesoderm to form blood have not been studied in detail for lack of molecular markers. We have studied the expression of the hematopoietic transcription factors GATA-1 and GATA-2 during Xenopus embryogenesis. During development GATA-1 expression is localized to the ventral region of the embryo and precedes the expression of embryonic globins. GATA-2 is highly expressed in the ventral region of the embryo by the end of gastrulation and later is expressed in the blood island region and the central nervous system. Lithium-induced dorsalization of embryos abrogates GATA-2 expression, and uv-induced ventralization of embryos leads to a radially symmetrical expression of GATA-2. Therefore, GATA-2 expression reflects the ventral character of the embryo. The expression of the GATA-binding proteins and globin in ventral marginal zone explants demonstrates that hematopoiesis is programmed as early as the blastula stage. GATA-1 and GATA-2 are also expressed in cultured animal cap explants, suggesting that these cells have hematopoietic potential. The developmental expression of GATA-1 and GATA-2 is consistent with their role in hematopoiesis in higher organisms and defines the ventral regions of the early embryo that give rise to hematopoietic progenitors. Our studies indicate that these genes will be useful in defining the inductive events that lead to the formation of hematopoietic mesoderm.