Object-Oriented (OO) database systems are becoming popular for CAD, VLSI, and CASE applications. One of the reasons is the support of prototyping through inheritance and/or schema evolution. In this paper, we propose another rapid prototyping technique: object prototyping. It assumes that a database schema is fixed and rapid prototyping is achieved by applying alternative values into attributes of an object. This is useful especially for design applications in which determining a design object's properties is not straightforward. We design a specification language based on the what-for-if construct for modeling the object prototyping. This specification language can 1) help the designer to simulate alternative design parameters to find out the optimal design parameters, 2) reduce the need for writing database programs, 3) support the development and construction of complex applications on top of the database, and 4) test the functionality and performance of design objects.