Cardiac hypertrophy(CH), a prominent feature that predisposes the heart to failure, is associated with the activation of multiple molecular and cellular changes in the circulation and heart. The Na+/H+ exchanger isoform 1 (NHE1) has been implicated in the development and progression of CH. NHE1 is an integral membrane protein that maintains intracellular pH by exchanging one intracellular H+ for one extracellular Na+. In various in vivo and in vitro models of cardiac dysfunction, NHE1 protein expression and activity have been shown to be upregulated. To better understand the involvement of NHE1, transgenic mice that express cardiac specific active NHE1 were studied (K-line mice). NHE1 activity of adult ventricular cardiomyocytes and protein expression were elevated by approximately 3-fold in the K-line mice vs. control. The K-line mice demonstrated significant global cardiac dysfunction. Left ventricular fractional cell shortening and ejection fraction were significantly decreased in the K-line mice (23.1 ± 3.8% and 45.2 ± 6.9% K-line vs. 36.5 ± 1.1% and 66.4 ± 1.5% control, respectively; P < 0.05). The K-line mice also exhibit myocardial remodeling. The heart weight to body weight ratio was significantly greater in the K-line mice (143 ± 10.0% of control; P < 0.05). Cross sectional area (K-line 195.6 ± 16.4% of control; P < 0.05) and interstitial fibrosis (K-line: 275.4 ± 11.6% of control; P < 0.05) were also elevated. Genechip analysis also revealed that expression of active NHE1 upregulated osteopontin (OPN) gene expression (>1,500 fold change) and its signaling pathways. OPN is a matricellular protein and a cytokine induced upon tissue injury and remodeling of various organs, including human heart failure. Our study shows that expression of activated NHE1 induces CH and elicits specific molecular changes that lead to CH.