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Spaceflight-associated neuro-ocular syndrome (SANS) involves unilateral or bilateral optic disc edema, widening of the optic nerve sheath, and posterior globe flattening. Owing to posterior globe flattening, it is hypothesized that microgravity causes a disproportionate change in intracranial pressure (ICP) relative to intraocular pressure. Countermeasures capable of reducing ICP include thigh cuffs and breathing against inspiratory resistance. Owing to the coupling of central venous pressure (CVP) and intracranial pressure, we hypothesized that both ICP and CVP will be reduced during both countermeasures. In four male participants (32 ± 13 yr) who were previously implanted with Ommaya reservoirs for treatment of unrelated clinical conditions, ICP was measured invasively through these ports. Subjects were healthy at the time of testing. CVP was measured invasively by a peripherally inserted central catheter. Participants breathed through an impedance threshold device (ITD, -7 cmH 2 O) to generate negative intrathoracic pressure for 5 min, and subsequently, wore bilateral thigh cuffs inflated to 30 mmHg for 2 min. Breathing through an ITD reduced both CVP (6 ± 2 vs. 3 ± 1 mmHg; P = 0.02) and ICP (16 ± 3 vs. 12 ± 1 mmHg; P = 0.04) compared to baseline, a result that was not observed during the free breathing condition (CVP, 6 ± 2 vs. 6 ± 2 mmHg, P = 0.87; ICP, 15 ± 3 vs. 15 ± 4 mmHg, P = 0.68). Inflation of the thigh cuffs to 30 mmHg caused no meaningful reduction in CVP in all four individuals (5 ± 4 vs. 5 ± 4 mmHg; P = 0.1), coincident with minimal reduction in ICP (15 ± 3 vs. 14 ± 4 mmHg; P = 0.13). The application of inspiratory resistance breathing resulted in reductions in both ICP and CVP, likely due to intrathoracic unloading. NEW & NOTEWORTHY Spaceflight causes pathological changes in the eye that may be due to the absence of gravitational unloading of intracranial pressure (ICP) under microgravity conditions commonly referred to as spaceflight-associated neuro-ocular syndrome (SANS), whereby countermeasures aimed at lowering ICP are necessary. These data show that impedance threshold breathing acutely reduces ICP via a reduction in central venous pressure (CVP). Whereas, acute thigh cuff inflation, a popular known spaceflight-associated countermeasure, had little effect on ICP and CVP.