At the cellular level, muscle provides a reservoir for body fluids to maintain fluid volume and blood pressure, so older adults may be at risk for hypotension due to loss of muscle with age. PURPOSE: To evaluate lean mass, hydration, and postural blood pressure in adults ≥ 65 years of age. METHODS: Older men (n=17) and women (n=30) completed two measurements of lean mass and hydration using multi-frequency bioelectrical impedance, and postural blood pressure lying, sitting, and standing. Day 1 was mid-day in a euhydrated state. Day 2 was the next morning, within 30 minutes of waking, in a fasted state. All were grouped for analysis by lean mass relative to height, using Lean Mass Index cut points of Low (women <14.9 kg/m2; men <18.7 kg/m2) and Normal (women ≥14.9 kg/m2; men ≥18.7 kg/m2). RESULTS: On Day 1, the Low group had lower relative lean mass (men 17.1 ± 0.4 vs. 20.2 ± 0.3 kg/m2; women 13.5 ± 0.2 vs. 16.5 ± 0.3 kg/m2; p < 0.001), absolute lean mass (men 55.7 ± 2.1 vs. 66.7 ± 1.8 kg; women 34.8 ± 0.9 vs. 44.3 ± 0.9 kg; p < 0.01), hydration (total body water: men 44.9 ± 1.0 vs. 51.1 ± 1.0 L; women 30.3 ± 0.5 vs. 36.1 ± 0.6 L; p < 0.001), and fluid volume (extracellular water: men 19.0 ± 0.4 vs. 22.2 ± 0.5L; women 14.4 ± 0.2 vs.17.2 ± 0.3 L; p <0.001) compared to the Normal group. Overnight (Day 2), both groups lost similar amounts of total body water (-0.83 ± 0.13 L; p < 0.001), extracellular water remained stable (0.01 ± 0.4 L), and the Low group preferentially lost more intramuscular water (-1.0 ± 0.8 vs. -0.4 ± 0.07 L). During postural changes from lying to standing the Low group had greater drops in systolic blood pressure (Day 1: -8.43 ± 2.8 vs. +2.29 ± 2.3 mmHg; p < 0.01; Day 2: -16.83 ± 2.7 vs. -2.83 ± 3.1 mmHg, p < 0.01). By comparison, diastolic blood pressure was more stable and compensated for postural changes from lying to standing in the Normal group, but not in the Low group (Day 1: Low +0.52 ± 1.7 vs. Normal +6.75 ± 1.4 mmHg, p < 0.01; Day 2: Low +0.35 ± 1.9 vs. Normal +5.58 ± 1.6 mmHg, p < 0.01). CONCLUSION: Loss of muscle with age is accompanied by loss of hydration and fluid volume that manifests as lower and less stable blood pressure. In fact, the severe drop in systolic blood pressure observed on Day 2 in the Low group approximates the criteria for diagnosis of orthostatic hypotension. Based on these data, loss of muscle increases risk for postural hypotension in older men and women.