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Cerebral blood flow (CBF) increases ~20% during whole body exercise although a Kety–Schmidt–determined CBF is reported to remain stable; a discrepancy that could reflect evaluation of arterial vs. internal jugular venous (IJV) flow and/or that CBF is influenced by posture. Here we test the hypothesis that IJV flow, as determined by retrograde thermodilution increases during exercise when body position is maintained.Introducing retrograde thermodilution, IJV flow was measured in eight healthy humans at supine and upright rest and during exercise in normoxia and hypoxia with results compared with changes in ultrasound-derived IJV flow and middle cerebral artery mean velocity (MCA Vmean).Thermodilution determined IJV flow was in reasonable agreement with values established in a phantom (R2 = 0.59, P < 0.0001) and correlated to the ultrasound-derived IJV flow (n = 7; Kendall τ, 0.28; P = 0.036). When subjects stood up, IJV blood flow decreased by 9% ± 13% (mean ± SD) (219 ± 57 to 191 ± 73 mL·min−1; P < 0.0001) and the influence of body position was maintained during exercise (P < 0.0001). Exercise increased both IJV flow and MCA Vmean (P = 0.019 and P = 0.012, respectively) and the two responses were similar (P = 0.50). During hypoxia, however, only MCA Vmean responded with a further increase (P < 0.0001).As determined by retrograde thermodilution, IJV flow seems little sensitive to hypoxia, but does demonstrate the about 15% reduction in CBF when humans are upright and, provided that body position is maintained, also the increase in CBF during whole body exercise.