Rapid prototyping of conventional user-machine interfaces offers the possibility of putting the human operator in the loop without the effort and cost associated with conventional simulation. Advocates of rapid prototyping suggest that it is compatible with conventional systems development techniques. It is not clear, however, exactly how rapid prototyping could be used in relation to conventional human factors engineering analyses. An investigation of the use of the VAPS rapid prototyping system was therefore carried out in six companies. The results show that a variety of analytical approaches can be used to initiate rapid prototyping. Overall, it appears that rapid prototyping facilitates an iterative approach to the development of the user-machine interface, and that is most applicable to the early stages of systems development, rather than to detailed design.