While significant progress has been made in understanding the molecular events underlying the early specification of the antero-posterior and dorso-ventral axes, little information is available regarding the cellular or molecular basis for left-right (LR) differences in animal morphogenesis. We describe the expression patterns of three genes involved in LR determination in chick embryos: activin receptor IIa, Sonic hedgehog (Shh), and cNR-1 (related to the mouse gene nodal). These genes are expressed asymmetrically during and after gastrulation and regulate the expression of one another in a sequential pathway. Moreover, manipulation of the sidedness of either activin protein or Shh expression alters heart situs. Together, these observations identify a cascade of molecular asymmetry in that determines morphological LR asymmetry in the chick embryo.