Impulse-conducting Purkinje cells differentiate from myocytes during embryogenesis. In the embryonic chicken heart, this conversion of contractile myocytes into conduction cells occurs subendocardially and periarterially. The unique sites of Purkinje fibre differentiation suggest that a shear stress-induced paracrine signal from the endocardium and arterial beds may induce adjacent myocytes to differentiate into conduction cells. Consistent with this model, Purkinje fibre marker genes can be induced in cultured embryonic myocytes by endothelin (ET), an endothelial cell-derived signalling peptide. This inductive response is, however, gradually lost from myocytes as embryos develop, and mature myocytes express only genes characteristic of hypertrophy in response to ET. In vivo, active ET is produced, through proteolytic processing, from its precursor by ET-converting enzyme 1 (ECE1) and triggers signalling by binding to its receptors, ETA and ETB. In the embryonic heart, the expression of these ET signalling components changes dynamically, defining the site and timing of Purkinje fibre differentiation within the ventricular myocardium during chick embryogenesis.