Automatic model generation is studied as part of a hybrid modeling strategy using simulation for performance analysis. Two major steps have to be carried out in this context. The program which is being investigated has to be translated into a model. During the translation, runtime has to be estimated for numerous computational blocks of statements which are replaced by simple delays. For performance estimation, the model has finally to be analyzed by an evaluation tool. Model evaluation as well as runtime estimation of computational blocks requires values of some variables, the control variables. We discuss the problem of automatic definition of control variables in general and consider some important cases. For the implementation of a model generating tool, we concentrate on parallel Fortran programs using message passing primitives for process communication.