ALK-1 is a type I serine/threonine kinase receptor for members of the TGF-beta superfamily of growth factors; its endogenous ligand is not known. In this study, we have analyzed the temporal and spatial expression pattern of ALK-1 mRNA in mouse embryos from the one-cell zygote until 12.5 dpc using RT-PCR and in situ hybridization. ALK-1 mRNA was first detected in the embryo at 6.5 dpc. From 7.5-8.5 dpc expression was highest at sites of vasculogenesis in both the embryonic and extraembryonic part of the conceptus, in trophoblast giant cells, and in the endothelial lining of the blood vessels in the decidua. From 9.5-12.5 dpc, ALK-1 was found to be expressed in several different tissues and organs, but was highest in blood vessels, mesenchyme of the lung, submucosal layer of the stomach and intestines, and at specific sites of epithelial-mesenchymal interactions. Its expression pattern suggests that ALK-1 is a type I receptor for TGF-beta1 in the developing mouse.