Age-associated changes in cardiac structure and function, together with estrogen loss, contribute to the progression of heart failure with preserved ejection fraction in older women. To investigate the effects of aging and estrogen loss on the development of its precursor, asymptomatic left ventricular diastolic dysfunction, echocardiograms were performed in 10 middle-aged (20 months) and 30 old-aged (30 months) female Fischer344×Brown-Norway rats, 4 and 8 weeks after ovariectomy (OVX) and sham procedures (gonads left intact). The cardioprotective potential of administering chronic G1, the selective agonist to the new G-protein-coupled estrogen receptor (GPER), was further evaluated in old rats (Old-OVX+G1) versus age-matched, vehicle-treated OVX and gonadal intact rats. Advanced age and estrogen loss led to decreases in myocardial relaxation and elevations in filling pressure, in part, due to reductions in phosphorylated phospholamban and increases in cardiac collagen deposition. Eight weeks of G-protein-coupled estrogen receptor activation in Old-OVX+G1 rats reversed the adverse effects of age and estrogen loss on myocardial relaxation through increases in sarcoplasmic reticulum Ca2+ ATPase expression and reductions in interstitial fibrosis. These findings may explain the preponderance of heart failure with preserved ejection fraction in older postmenopausal women and provide a promising, late-life therapeutic target to reverse or halt the progression of left ventricular diastolic dysfunction.