Abstract Body: Non-dipping of blood pressure (BP) during the nighttime is an independent risk factor for hypertension. Importantly, BP dipping is diminished in postmenopausal women, suggesting that the female sex hormones may contribute to the circadian rhythm of BP. Estradiol, activates the G protein-coupled estrogen receptor 1 (GPER1), which elicits protective actions within the cardiovascular system. However, little is known about the GPER1 effect on circadian BP regulation in aged females. Taken together, we hypothesized that systemic GPER1 activation lowers BP and regulates circadian rhythm of BP in aged females. To test the hypothesis, 23-24 months-old C57BL/6J female mice (n=6) were implanted with radio-telemeters for BP recording. After 21 days of postsurgical recovery, BP was recorded. Mice were then given vehicle intraperitoneally daily for 3 consecutive days, followed by the selective GPER1 agonist (LNS8801, 400 µg/kg/day) for an additional 3 days. Data were collected over 24-hour periods according to the 12-hour active period and the 12-hour inactive period. GPER1 activation lowered 24-hour mean arterial pressure (MAP) compared to vehicle control values (113±3 vs. 117±3 mmHg; p=0.01). Similar to MAP, 24-hour systolic blood pressure (SBP) and diastolic blood pressure (DBP) were lower in LNS8801-treated mice compared to corresponding vehicle-treated mice. However, there was not a significant difference in 24-hour heart rate (HR). Interestingly, MAP during the inactive period was lower in LNS8801-treated mice compared to corresponding vehicle-treated mice (104±2 vs. 110±3, respectively, p=0.02). Whereas, GPER1 activation did not elicit significant changes in MAP during the active period. Similarly, LNS8801 reduced DBP during the inactive period (88±4 vs. 96±4 mmHg; p=0.04). We extended the study in aged female mice (14–16 months) with genetic deletion of GPER1 (GPER1 KO; n=6) and wildtype littermates (WT; n=5). Genetic deletion of GPER1 increased 24-hour MAP, SBP, and DBP, whereas HR was not significantly different between genotypes. Noteworthy, GPER1 KO increased MAP during the active period (126±2 vs. 114±1 mmHg; p=0.009) and during the inactive period, compared to WT (110±3 vs. 101±1 mmHg; p=0.01). The results indicate that GPER1 activity lowers BP. Interestingly, LNS8801 acutely improves BP dipping during the inactive period. Overall, GPER1 activation elicits BP-lowering effects in aged female mice, possibly by regulating the circadian BP rhythm.