Lower body negative pressure (LBNP), an experimental model of hemorrhage (Hem), causes shifts in blood volume to the legs and elicits central hypovolemia. This study defined the levels of LBNP which equate to actual Hem. Male baboons (n=10) were sedated and instrumented for measurement of arterial pressure (AP), pulse pressure (PP), and central venous pressure (CVP). Blood was removed in four steps (7 min/step): 6.25%, 12.5%, 18.75%, and 25% of total blood volume. Shed blood was returned and the animals were allowed to recover. Four weeks after the Hem protocol, the same animals were subjected to four levels of LBNP which elicited equivalent changes in PP and/or CVP observed during Hem. Blood samples were collected before Hem or LBNP( baseline, B), after the second Hem step or LBNP (midpoint, MP), and after the fourth Hem step or LBNP (pre‐syncope, PS). Blood samples were analyzed for plasma renin activity (PRA), catecholamines ( Nepi, Epi) and vasopressin (AVP). The results for PRA (mean±sem) are shown below: PRA (ng AngI/ml h) Study B MP SP Hem 2.8±0.7 3.9±0.9 7.9±1.8 LBNP 4.1±1.0 5.4±1.7 9.9±2.7 Repeated measures ANOVA of the log transformed PRA data indicated that PRA was significantly increased relative to B for the MP and PS samples and that PS was higher than MP for both the Hem and LBNP studies. However, there was no significant difference between Hem and LBNP for PRA at the B, MP and SP time points. Similar results were obtained for plasma Nepi, Epi and AVP. We conclude that LBNP levels can be used as surrogates for actual blood loss. This study supports the validity of LBNP as a human model of central hypovolemia associated with Hem. Funding Source: U.S. Army MRMC Combat Casualty Care Research Program.