EXPOSURE TO WEIGHTLESSNESS RESULTS IN THE REMOVAL OF HYDROSTATIC PRESSURE GRADIENTS AND A HEADWARD FLUID SHIFT. THIS HAS CURRENTLY UNKNOWN CONSEQUENCES, BUT IT HAS BEEN HYPOTHESIZED TO CONTRIBUTE TO A SERIES OF NEURO-OCULAR STRUCTURAL AND FUNCTIONAL CHANGES OCCURRING DURING SPACEFLIGHT, COLLECTIVELY KNOWN AS SPACEFLIGHT ASSOCIATED NEURO-OCULAR SYNDROME (SANS). ADDITIONALLY, RECENTLY DEMONSTRATED STAGNANT AND RETROGRADE BLOOD FLOW AND VENOUS THROMBOSIS IN THE LEFT INTERNAL JUGULAR VEIN (IJV) DURING SPACEFLIGHT COULD ALSO BE ASSOCIATED WITH SUSTAINED HEADWARD FLUID SHIFT AND REMOVAL OF HYDROSTATIC GRADIENTS. THUS, COUNTERMEASURES FOCUSED ON REVERSING THE HEADWARD FLUID SHIFT, SUCH AS LOWER BODY NEGATIVE PRESSURE (LBNP), BECOME PARTICULARLY RELEVANT. PREVIOUS STUDIES ON THE GROUND (I.E., EARTH CONDITIONS, MOSTLY IN SUPINE POSITION OR 6 HEAD-DOWN TILT) INVESTIGATING THE EFFECTS OF LBNP HAVE SHOWN PROMISING RESULTS. HOWEVER, THERE ARE FUNDAMENTAL DIFFERENCES IN PHYSIOLOGICAL RESPONSES BETWEEN EARTH AND MICROGRAVITY CONDITIONS (E.G., PRESENCE/ABSENCE OF FRONT-TO-BACK, OR GX, ACCELERATION). THUS, GROUND RESULTS MIGHT NOT BE DIRECTLY EXTRAPOLATED TO TRUE MICROGRAVITY CONDITIONS. THE OBJECTIVE OF THIS ANALOG-BASED RESEARCH EFFORT IS TO GENERATE ACUTE DOSE-RESPONSE CURVES OF A LARGE SET OF CARDIOVASCULAR, AUTONOMIC, CEPHALAD, AND OCULAR MEASURES AS A FUNCTION OF LBNP LEVEL DURING TRUE MICROGRAVITY CONDITIONS USING PARABOLIC FLIGHT.