The widespread use of power wheelchairs has greatly increased the requirements for the accessibility of buildings and other architectural structures to handicapped persons. In addition, recent advances in microcomputer technology have made possible increasingly sophisticated power wheelchair interfaces, such as halo, puff and sip, and muscle control mechanisms, which can provide mobility for an even larger portion of the handicapped population. Finally, the ADA (the Americans with Disabilities Act of 1990) requires handicapped accessibility for (almost) all public structures. We have developed a virtual structure prototyping system that allows navigation by a person using a power wheelchair. The system is a tool for three groups of people: (1) for architects and designers, it provides structure visualization that can both improve the handicapped accessibility of building designs and test a structure for ADA compliance; (2) for wheelchair users, it provides more appropriate device fitting and training with wheelchair control mechanisms; and (3) for health care professionals, it provides evaluations of wheelchair users. The system consists of an instrumented, joystick-driven power wheelchair connected to a high-performance graphics workstation; it simulates the actual speed and maneuverability of the particular wheelchair within a virtual structure. The display generates realistic interiors containing multiple light sources and surface textures and is viewed in stereo through lightweight polarized glasses. The system maintains a hierarchical data structure which detects collisions between the virtual wheelchair and the environment. In this paper we discuss (1) the system's user interface, (2) the system's hardware and software configuration, (3) the impact of the system on the architectural design process, and (4) future system additions. In the last section we also discuss virtual manipulation for enabling technology.