['Neuronal calcium sensor-1 (NCS-1) regulates various neuronal functions. Although it is expressed in the heart, very little is known about its cardiac functions.', 'This study aimed to identify the physiological and pathological roles of NCS-1 in the heart.', 'We characterized the cardiac functions of knockout mice (Ncs1(-/-)) and identified NCS-1 as a novel regulator of cardiac Ca(2+) signaling, specifically in immature and hypertrophic hearts. NCS-1 was highly expressed in young hearts, and its deletion decreased survival and contractile function in young mice. Intracellular Ca(2+) levels and sarcoplasmic reticulum Ca(2+) content were significantly lower in Ncs1(-/-) myocytes than in wild-type cells. This was due to reduced Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) activity in Ncs1(-/-) myocytes, which led to reduced sarcoplasmic reticulum Ca(2+) uptake and release. NCS-1 physically and functionally interacted with inositol 1,4,5-trisphosphate receptors (IP(3)Rs) in the heart. In addition, IP(3)R stimulation resulted in phosphorylation of CaMKII-δ, which was enhanced by NCS-1 overexpression. These results suggest that a functional link exists between NCS-1, IP(3)R function, and CaMKII activation that may affect global Ca(2+) signals in the immature heart. Furthermore, NCS-1 was upregulated in hypertrophic hearts, and hormone-induced hypertrophy was largely prevented in Ncs1(-/-) hearts. Inhibitors of IP(3)Rs, CaMKII, and calcineurin all prevented NCS-1-induced hypertrophy, which suggests the involvement of these pathways.'] Does neuronal calcium sensor-1 promote immature heart function and hypertrophy by enhancing Ca2+ signals?