Abstract: Object-oriented modeling languages, tools, and methods more and more attract the interest of embedded (real-time) system developers. This is especially true if embedded (real-time) system software has to cooperate with interactive multimedia software, as it is more and more the case in automotive systems. It is still an open question whether and how the standard OO modeling language UML and its accompanying tools have to be adapted to the regarded application domain. This paper evaluates the development of a rapid prototype for an air condition controller with the popular CASE tool Rational Rose/RT(r). We point out some weaknesses of the presented solution and propose an extension to Rose/RT(r), which overcomes the weaknesses by combining Rose/RT's UML dialect with data flow equations.