Introduction: MicroRNA-1 (miR1) is an evolutionarily conserved, striated muscle-enriched microRNA and plays critical roles in regulation of the development and physiology of the heart. It has been found that miR1 was downregulated in various types of cardiac diseases. We observed that, compared to that in young mice (age 3-4 months), the expression of miR1 was significantly decreased to the level of 12.5±4.0% in the heart of aged animals (age 18 months). Hypothesis: We hypothesized that miR1 deficiency is a major etiology in diseased and/or aged hearts. In the mammalian genome, two distinct genes, miR1-1 and miR1-2 that are located on two chromosomes, generate identical mature miR1. Methods and Results: To study the hypothesis, we generated miR1-75% knockdown (75%KD) mice by crossbreeding miR1-1 knockout (KO) and miR1-2 KO animals and found that miR1-75%KD mice could survive to adult age with a relatively higher mortality, while miR1-100%-KO is postnatal lethal. The ejection fraction and fraction shortening of 75%KD hearts was significantly decreased than that of WT animals. Electrocardiogram showed that, compared to the WT group, 75%KD hearts had significantly slower heart rates with shorter PR interval and prolonged QRS interval with observed premature ventricular contractions. Optical mapping of ex vivo hearts revealed significantly slower ventricular conduction velocity in 75%KD hearts. Compared to WT cells, 75%KD ventricular cardiomyocytes had hyperpolarized RMPs with prolonged action potentials, and showed significantly decreased sarcomere shortening. Proteomic assays found that proteins of mitochondrial ATP synthesis and electron transport were significantly downregulated in 75%KD hearts. Conclusions: We conclude that the deficiency of miR1 is a major etiological factor of cardiac electrical and metabolic remodeling in the heart. Restoring the normal level of miR1 in the heart could be a critical and effective therapeutic strategy to prevent and/or treat heart diseases.