Athletes who dehydrate voluntarily for weight-class sports and then engage in weight training may place themselves at risk for orthostatic instability. Purpose. To test the hypothesis that voluntary dehydration exacerbates reductions of cerebral blood velocity (CBV) and impairs cerebral autoregulation during standing after heavy resistance exercise. Methods. We studied 10 males (24±2 yrs, 174±6 cm, 77±7 kg). Subjects were randomly assigned to either maintain euhydration (EUH) throughout the day, or voluntarily refrain from consuming any liquid in combination with ingestion of 40 mg furosemide (dehydration; DEH). Each subject participated twice, under both conditions (crossover). Changes in blood and plasma volume were calculated from hemoglobin and hematocrit. We recorded the ECG, finger photoplethysmographic arterial pressure (Finometer), cerebral blood velocity (transcranial Doppler), and end-tidal CO₂ during two sets of 10 repetitions of recumbent leg press exercise at 85% of subjects' predicted 6 repetition maximum, and then during 10 min standing immediately after exercise. Data were analyzed in both time and frequency domains. Cross-spectral coherence and transfer function gain between arterial pulse pressure and CBV pulse served as indicators of cerebral autoregulation. Results. Dehydration reduced plasma volume by 11%. Arterial pressure and CBV increased during exercise independent of hydration status. During standing after exercise heart rates were significantly higher, and CBV lower with DEH. Coherence was not different between conditions, but transfer function gain decreased significantly during the last 5 min of standing for EUH. Conclusions. Reductions of transfer function gain (indicating increased autoregulatory capacity) during standing under EUH, but not DEH, suggests that DEH represents a significant challenge to cerebral autoregulatory mechanisms. Failure to improve autoregulatory capacity with DEH may contribute to the observed lower CBV measured in DEH subjects during standing after heavy resistance exercise.