In mouse embryos, the heart begins beating and a network of blood vessels forms in the yolk sac early in development. These initially simple structures are then extensively remodelled. Vascular remodelling needs a normal blood flow, but does this flow deliver oxygen, signalling molecules or mechanical cues? On p. 3317,Lucitti and co-workers report that haemodynamic force drives vascular remodelling in mouse embryos. First, they use time-lapse confocal microscopy and fluorescence recovery after photobleaching (FRAP) to examine blood flow in normal embryos and in embryos lacking myosin light chain 2a (in which impaired cardiac contractility prevents vessel remodelling). These experiments indicate that erythroblast entry into the circulation triggers vessel remodelling. Next, the researchers show that using acrylamide to retain erythroblasts in embryonic blood islands in vivo reduces shear stress and prevents vessel remodelling, which is restored by the addition of a colloid that expands plasma volume. Thus, haemodynamic force is necessary and sufficient to induce vascular remodelling in the mammalian yolk sac.