Increasing evidence suggests that blood flow and biomechanical forces generated by the developing heart feedback to regulate cardiac chamber formation and maturation. How such forces are sensed and translated, however, remains unclear. Here, Jiandong Liu and co-workers show that, in zebrafish, cardiac contraction activates Notch signalling in the endocardium to control chamber maturation (p. 4080). By analysing embryos deficient for troponin T type 2a (tnnt2a), which encodes an essential component of the cardiac contraction apparatus, the researchers first show that cardiac contraction is required for the formation of cardiac trabeculae – the luminal protrusions that are indicative of cardiac chamber maturation. They further show that cardiac contraction controls Notch signalling in the endocardium; notch1b expression is reduced in tnnt2a–/– embryos. Notch activation, they report, induces the expression of ephrin b2a and neuregulin 1 to promote trabeculation. Finally, the authors demonstrate that shear stress controls notch1b expression in a primary cilia-dependent manner, suggesting that primary cilia in this context are responsible for detecting fluid flow. In summary, these findings highlight a molecular mechanism that links flow sensing to the transcriptional changes that regulate cardiac development.