Intracardiac flow patterns during heart development were studied by injection of india ink into the yolk sac circulation of chick embryos at Hamburger-Hamilton stages 10 to 17. We injected india ink into a small venule or capillary, carefully preventing application of overpressure to the vascular system, and recorded the intracardiac route by video. From stage 12 onward, blood flow was laminar, and separate intracardiac currents were visualized. The yolk sac was divided into a left and a right half. Blood coursed through each half in concentric loops, ranging from the marginal sinus to the sinus venosus. This parallel array persisted within the heart. Bilateral to the embryo, two lateral regions arose that extended wedge-like within each half, resulting in six equally sized yolk sac regions at stage 16. The process of heart looping was not accompanied by a change in flow pattern. However, developmental changes of the yolk sac circulation were reflected in alteration of the intracardiac flow pattern. From stage 16 onward, the intracardiac flow pattern was no longer determined by the left- or right-hand side of the yolk sac but by bilateral anterior, lateral, and posterior regions of the yolk sac. Blood from the lateral regions of the yolk sac was preferentially distributed to the head. The results show that in preseptation stages a relatively stable flow pattern is present. We suggest that alterations in blood flow could influence the process of normal heart development.