In-flight lower body negative pressure (LBNP) transiently restores normal body fluid distribution.  Early in the U.S. space program, LBNP was devised as a way to test for orthostatic intolerance.  LBNP was also investigated not only as an in-flight cardiovascular orthostatic stress test, but also as a countermeasure to cardiovascular deconditioning on Soviet space stations, Skylab, and the Shuttle. 
LBNP
With the development of the Skylab Program and longer duration spaceflight, it was realized that it could provide a method of monitoring orthostatic intolerance in flight and predicting the post-landing orthostatic response. 
Skylab Program
It is still being used by the Russian program on the International Space Station as an end-of-flight countermeasure.
Russian program
It is still being used by the Russian program on the International Space Station as an end-of-flight countermeasure.
International Space Station