Steady‐state tilt has no effect on cerebrovascular reactivity to increases in the partial pressure of end‐tidal carbon dioxide (PETCO 2). However, the anterior and posterior cerebral circulations may respond differently to a variety of stimuli that alter central blood volume, including lower body negative pressure (LBNP). Little is known about the superimposed effects of head‐up tilt (HUT; decreased central blood volume and intracranial pressure) and head‐down tilt (HDT; increased central blood volume and intracranial pressure), and LBNP on cerebral blood flow (CBF) responses. We hypothesized that (a) cerebral blood velocity (CBV; an index of CBF) responses during LBNP would not change with HUT and HDT, and (b) CBV in the anterior cerebral circulation would decrease to a greater extent compared to posterior CBV during LBNP when controlling PETCO 2. In 13 male participants, we measured CBV in the anterior (middle cerebral artery, style="fixed-case">MCAv) and posterior (posterior cerebral artery, style="fixed-case">PCAv) cerebral circulations using transcranial Doppler ultrasound during style="fixed-case">LBNP stress (−50 mmHg) in three body positions (45° style="fixed-case">HUT, supine, 45° style="fixed-case">HDT). style="fixed-case">PETCO 2 was measured continuously and maintained at constant levels during style="fixed-case">LBNP through coached breathing. Our main findings were that (a) steady‐state tilt had no effect on style="fixed-case">CBV responses during style="fixed-case">LBNP in both the style="fixed-case">MCA (P = 0.077) and style="fixed-case">PCA (P = 0.583), and (b) despite controlling for style="fixed-case">PETCO 2, both the style="fixed-case">MCAv and style="fixed-case">PCAv decreased by the same magnitude during style="fixed-case">LBNP in style="fixed-case">HUT (P = 0.348), supine (P = 0.694), and style="fixed-case">HDT (P = 0.407). Here, we demonstrate that there are no differences in anterior and posterior circulations in response to style="fixed-case">LBNP in different body positions.
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