Nodal is a member of the transforming growth factor β (TGF-β) superfamily that plays critical roles during embyogenesis. Recently, we have demonstrated that Nodal induces apoptosis and inhibits proliferation in human trophoblast and epithelial ovarian cancer cells. To further determine the role of Nodal in controlling cellular activities, we examined the action of Nodal and its type I receptor, activin receptor-like kinase 7 (ALK7), on breast cancer cell lines. Using RT-PCR, we detected Nodal and ALK7 transcripts in MDA-MB-231 and MCF-7 cells. Overexpression of Nodal or activation of constitutively active ALK7 (ALK7-ca) resulted in a significant decrease in the number of live cells and a significant increase in the number of dead cells. This effect was observed for both cell lines; however, Nodal and ALK7-ca had a much stronger effect in MDA-MB-231 cells than in MCF-7 cells. The effect of Nodal was blocked by dominant negative mutants of ALK7, suggesting that Nodal acts through ALK7 to inhibit cell growth/survival. Nodal and ALK7-ca inhibited proliferation in both cell lines; however, while Nodal and ALK7-ca induced apoptosis in MDA-MB-231 cells, they only had a minor effect on MCF-7 cells. In addition, Nodal activated caspase 3 in MDA-MB-231 cells, but had no effect on caspase 3-deficient MCF-7 cells. The effect of Nodal on apoptosis in MDA-MB-231 cells was blocked by a caspase 3 inhibitor. These findings demonstrate that the Nodal-ALK7 pathway exerts anti-proliferative and proapoptotic effects in breast cancer cells and suggest that caspase 3 is important for Nodal-ALK7-induced apoptosis.