Blood pressure changes in response to the Valsalva maneuver (VM), which reflect the integrity of the baroreflex that regulates blood pressure. Performing this maneuver in the standard supine position often prevents adequate venous preload reduction, resulting in a rise rather than a fall in blood pressure, the "flat-top" Valsalva response. We determined whether performing the VM at a 20 degree angle of head-up tilt (20 degrees ) improves preload reduction, thereby reducing the frequency of flat-top responses, improving reflex vasoconstriction, and increasing the Valsalva ratio. One hundred thirty patients were evaluated in a prospective study. Each patient performed the VM in both supine and 20 degrees positions.Flat-top responses were present in 18% of subjects when supine. Twenty degree angle of head-up tilt position reduced the flat-top response by 87%. The components of the response that are dependent on preload reduction (Valsalva ratio and phases II_E, II_L, and IV) also showed significant improvement with 20 degrees .A 20 degree angle of tilt is sufficient to reduce venous preload, decreasing flat-top response rate and improving the Valsalva ratio and the morphology of the VM. We recommend this modification for laboratory evaluation of the VM, whenever a flat-top response is seen.