Background: Cerebral autoregulation is the mechanism that maintains adequate cerebral blood flow despite fluctuations in systemic blood pressure (BP) and/or volume. To date, this has primarily been assessed using transcranial Doppler of the middle cerebral artery, which provides measures of blood velocity in one vessel. The aim of this study was to use high-resolution magnetic resonance imaging (MRI) to examine whether autoregulation of whole brain blood flow is maintained during shifts in blood volume due to lower body negative pressure (LBNP) in healthy middle-aged volunteers. This is the first study to use MRI to measure brain blood flow during LBNP. Methods: 11 healthy volunteers (4 females), age 50.9±0.7 years (mean ± SEM), body mass index 26.3±0.2 kg/m2, ambulatory day time blood pressure 122/80 ±0.6/0.6 mmHg) were recruited. Cardiac-gated phase contrast MRI at 1.5T (Siemens Avanto) was completed to measure cerebral blood flow. Images were obtained in the transverse plane perpendicular to the internal carotid arteries at the level of the basilar artery. Passive changes in central blood volume were induced by graded LBNP at -20, -40 and -50mmHg. Participants were maintained at each level of LBNP for 1 minute before imaging. The internal carotid and basilar arteries were contoured in each image; mean flow velocity and blood flow were quantified in each vessel using semi-automated software (Argus, Siemens Healthcare, UK). The data was analysed using repeated measures one-way analysis of variance (ANOVA) and post-hoc using Bonferroni multiple comparisons with respect to resting values. Correlations were calculated using Pearson’s correlation coefficient. Results: Brain blood flow and mean velocity were decreased during lower body negative pressure greater than -20mmHg (Figure 1a and 1b, p<0.0001 for both, ANOVA). Diastolic BP (Figure 1c, p=0.0025) and heart rate (Figure 1d, p<0.0001) showed an increase at high levels of LBNP, while there was no difference in the systolic or mean BP during LBNP (p=0.07, p=0.48, respectively). Importantly, there were no reported alterations in the state of consciousness or development of presyncopal symptoms in any of the participants. There were no correlations of BMI (P>0.05), ambulatory systolic BP (P>0.05) or age (P>0.05) to brain blood flow at any level of LBNP. Conclusion: Total cerebral blood flow and blood flow velocity is reduced in response to central hypovolemia before changes in systemic blood pressure occur. This indicates that cerebral blood flow is not controlled within narrow limits during simulated hypovolaemia in healthy middle-aged participants, despite well-regulated blood pressure.