Abstract Body: Introduction: Heart failure (HF) is a growing health burden in the United States. Although there is no consensus about sex differences in HF with mildly reduced ejection fraction (HFmrEF) prevalence, HF with preserved ejection fraction (HFpEF) is prevalent in women, and it is aggravated by endothelial cell (EC) microvascular rarefaction. Pathological cardiac remodeling can alter the mechanoregulatory properties of the heart, and severe dilated cardiomyopathy is observed with dysruption of cardiac Piezo1. We hypothesized that loss of cardiac Piezo1 and EC rarefaction contribute to the development of HFmrEF in male, but not female, hypertensive BPH/2J mice, and aimed to investigate whether dysregulation of Piezo1 in the heart contributes differently to the progression of HF in males and females. Methods: Male and female BPN/3J and BPH/2J mice (1.5-year-old) were used and their blood pressure measured via radiotelemetry. Cardiac function (left vetricular ejection fraction – LVEF, mitral valve E/A, and E/e’) was assessed by echocardiography. Cardiac Piezo1 and EC rarefaction were evaluated in the LV by Western blotting and immunofluorescence. Data are shown as mean±S.E.M., and analyzed by Two-way ANOVA, with Tukey post-hoc (p<0.05). Results: Male and female BPH/2J mice were hypertensive (systolic blood pressure BPN/3J male: 127.1±1.4mmHg; female: 115.6±1.9mmHg; BPH/2J male: 158.0±1.7mmHg^#; female: 152.7±2.5mmHg^#; n=4 p<0.05). Specifically, male BPH/2J mice showed HFmrEF (LVEF male BPN/3J: 69.43±3.56%; BPH/2J: 43.28±1.21%^#; E/A male BPN/3J: 1.94±0.15; BPH/2J: 2.12±0.26; E/e’ male BPN/3J: 39.01±1.22; BPH/2J: 54.76±4.03^#; n=6-14 p<0.05), and female BPH/2J developed HFpEF (LVEF female BPN/3J: 63.38±5.60%; BPH/2J: 51.35±3.13%^#; E/A female BPN/3J: 2.32±0.25; BPH/2J: 3.83±0.60^#; E/e’ female BPN/3J: 32.73±4.57; BPH/2J: 39.24±3.53; n=6-11 p<0.05). Loss of cardiac Piezo1 was detected in male BPH/2J (BPN/3J: 0.62±0.02U; BPH/2J: 0.43±0.03U^#; n=5-6 p<0.05), while Piezo1 channels were upregulated in female BPH/2J (BPN/3J: 0.55±0.06U; BPH/2J: 0.83±0.02U^#; n=5 p<0.05). Cardiac EC rarefaction was only observed in male BPH/2J (BPN/3J male: 7.36±0.31%; BPH/2J male: 5.20±0.36%^#; n=8 p<0.05). Conclusion: These results reveal for the first time that male and female BPH/2J mice spontaneously develop HF induced by hypertension, later in life. However, loss of cardiac Piezo1 and EC microvascular rarefaction may contribute to the sex-dependent outcomes of HF in BPH/2J mice.