The ability to generate homogeneous intracellular Ca2+ oscillations at high frequency is the basis of the rhythmic contractions of mammalian cardiac myocytes. While the specific mechanisms and structures enabling homogeneous high-frequency Ca2+ signals in adult cardiomyocytes are well characterized, it is not known how these kind of Ca2+ signals are produced in developing cardiomyocytes. Here we investigated the mechanisms reducing spatial and temporal heterogeneity of cytosolic Ca2+ signals in mouse embryonic ventricular cardiomyocytes. We show that in developing cardiomyocytes the propagating Ca2+ signals are amplified in cytosol by local Ca2+ releases. Local releases are based on regular 3-D sarcoplasmic reticulum (SR) structures containing SR Ca2+ uptake ATPases (SERCA) and Ca2+ release channels (ryanodine receptors, RyRs) at regular intervals throughout the cytosol. By evoking [Ca2+]i-induced Ca2+ sparks, the local release sites promote a 3-fold increase in the cytosolic Ca2+ propagation speed. We further demonstrate by mathematical modelling that without these local release sites the developing cardiomyocytes lose their ability to generate homogeneous global Ca2+ signals at a sufficiently high frequency. The mechanism described here is robust and indispensable for normal mammalian cardiomyocyte function from the first heartbeats during the early embryonic phase till terminal differentiation after birth. These results suggest that local cytosolic Ca2+ releases are indispensable for normal cardiomyocyte development and function of developing heart.