Prototype design and testing is an indispensable stage of any project development in many fields of activity, such as aeronautical, spatial, automotive industries or architecture. Scientists and engineers rely on prototyping for a visual confirmation and validation of both their ideas and concepts. This paper describes the design and implementation of PROVIS, a system for prototyping virtual systems in a collaborative way. The main goal of our works is to allow designers to replace physical mock-ups by virtual ones in order to test the integration and space requirements of the model components. To this end, PROVIS combines a "desktop VR" interface with an interactive 3D display. While discussing its software and hardware architecture, the paper describes how PROVIS allows users to interact with flexible or articulated objects, and to test maintenance procedures and component accessibility thanks to an original use of genetic algorithms.