We analyzed the extracellular matrix and the connective tissue of the developing chick myocardium ("myocardial interstitium"). The importance of this myocardial element for heart function has been well documented both for the normal and pathologic adult hearts. However, little information is available on the organization of the embryonic myocardial interstitium and its modifications during development and increasing intracardiac pressure. In the present study we used light and scanning electron microscopic techniques, and lectin probes to study the interstitium of the ventricular myocardium of chick embryos from stage 29 (day 6 of development) until hatching. Our observations trace the progressive appearance and organization of the elements of the extracellular matrix, comprising the epimysium, perimysium and endomysium, which form a well-defined architectural network. Finally, we discuss the role of these elements of the extracellular matrix and their possible relation with the biomechanical properties of developing heart.