The long term goal of the studies proposed is to understand the role of activin receptor signaling in mammalian embryonic development. This understanding will be gained through the following three aims: (1) Generation of a conditional "knock-out" allele for the activin receptor ActR-IIA and analysis of the efficiency of this alleles in vivo. (2) Examination of cardiac histology and cardiac specific gene expression in embryos in which ActRIIA function has been conditional removed from the hearts of ActRIIB null embryos. (3) Determinations of the ligand that utilizes ActRIIA and ActRIIB during cardiogenesis. Activin receptors can transduce signals that regulate a variety of cell responses including regulating the growth and differentiation of embryonal carcinoma cells, induction of mesodermal cell types in amphibian embryo explants, and regulation of the cell cycles. The outcome of these studies with the activin receptors should provide a greater understanding of the role of activin receptor signaling in mammalian development.