This paper describes an approach to neuroscientific reaction-time experiments and resulting methods for data processing under real-time constraints in virtual environments (VE). Immersive VEs produce huge amounts of data, which has to be processed in real-time to meet the interactivity requirement of VE. Furthermore, data is needed often with a timing resolution and even more crucial with an accuracy in the range of milliseconds. Unfortunately recent operating systems do not in general guarantee timing precision in this range. For these purposes we have researched possibilities for reaction-time measurement and real-time data acquisition. Our system enables neuroscientists to perform reaction-time experiments in platform-independent virtual environments.