Retinoic acid (RA) has recently been implicated in the establishment of the correct anteroposterior (AP) polarity in the developing heart, which is essential if the primitive circulatory pump is to couple efficiently with pre-existing blood vessels. Now, on p. 5363, Hochgreb et al. describe a two-step model for the involvement of RA in cardiac AP polarity determination, based on in situ hybridisation, morphometry, fate-mapping and RA antagonist treatment studies of chick embryos. They suggest that early in heart development, cardiac precursors are specified to a sino-atrial fate by exposure to a low concentration of RA produced by retinaldehyde dehydrogenase 2 (RALDH2) expression in the lateral mesoderm. Subsequently, a caudorostral wave of RALDH2 expression in the anterior lateral mesoderm provides an increase in RA concentration that, they propose, determines the AP fate of the cardiac precursor cells.