The asymmetries of internal organs are consistently oriented along the left-right axis in all vertebrates, and perturbations of left-right orientation lead to significant congenital disease. We propose a model in which a "left-right coordinator" interacts with the Spemann organizer to coordinate the evolutionarily conserved three-dimensional asymmetries in the embryo. The Vg1 cell-signaling pathway plays a central role in left-right coordinator function. Antagonists of Vg1 alter left-right development; antagonists of other members of the TGFbeta family do not. Cell-lineage directed expression of Vg1 protein can fully invert the left-right axis (situs inversus), can randomize left-right asymmetries, or can "rescue" a perturbed left-right axis in conjoined twins to normal orientation (situs solitus), indicating that Vg1 can mimic left-right coordinator activity. These are the first molecular manipulations in any vertebrate by which the left-right axis can be reliably controlled.