INTRODUCTION: Exposure to high altitude results in a loss in plasma volume (PV) which augments hemoglobin (Hb) and thus arterial oxygen content (CaO2). Conversely, heat acclimation (HA) results in plasma volume (PV) expansion. However, it is unclear whether HA-induced PV expansion impacts the change in PV and CaO2 during a hypobaric hypoxia (HH) exposure. PURPOSE: To determine whether HA-induced PV expansion alters CaO2 during HH exposure. METHODS: Thirteen sea-level natives (mean ± SD; men, age: 21 ± 3 yr; ht: 172 ± 0.8 cm; wt: 76 ± 13 kg; sea level VO2peak 42.9 ± 4.6 ml·kg-1·min-1) participated in a three phase study consisting of a: 1) 30 hour HH exposure (HH1; 3,500 m, 495mmHg), 2) 14-day washout period in which volunteers completed an 8-day exercise-HA protocol (treadmill walking: 5 km·hr-1, 2% grade; 40°C and 40% RH), and 3) second 30 hour HH exposure (HH2). After 20 minutes of seated postural control, venous and arterialized capillary blood samples were collected pre- and post-HA and after 22 hours of HH1 and HH2. Samples were analyzed for Hb, Hct, arterialized oxygen saturation (SaO2), and arterialized partial pressure of oxygen (PaO2). The percent change in plasma volume (%ΔPV) was calculated in accordance with the Dill and Costill equation while CaO2 was calculated using the following formula: CaO2 = (1.34*Hb* SaO2/100) + (0.0031*PaO2). A t-test was used to assess differences in the %ΔPV, Hb, SaO2, PaO2, and CaO2 between HH1 and HH2. RESULTS: The %ΔPV from pre- to post-HA was 7 ± 9%. From HH1 to HH2, the %ΔPV decreased to a greater extent (-12 ± 3 vs -17 ± 5; P < 0.01). From HH1 to HH2, Hb (16.1 ± 0.9 vs 16.0 ± 0.9 mg/dl), SaO2 (89 ± 4% vs 90 ± 4%), PaO2 (56.7 ± 2.8 vs 57.6 ± 3.2 mmHg) and CaO2 (19.4 ± 1.1 vs 19.4 ± 1.3 ml/l) were not different (P > 0.05). CONCLUSION: Our results suggests that although HA induced a PV expansion; and there was a greater change in PV during HH2, CaO2 was not affected. Supported by USAMRDC; author views not official US Army or DoD policy