  A model is developed for the hadronic interaction in the two-nucleon system
above pion threshold which is based on meson, nucleon and $\Delta$ degrees of
freedom and which includes full meson retardation in the exchange operators.
For technical reasons, the model allows maximal one meson to be present
explicitly. Thus the Hilbert space contains besides $NN$ and $N\Delta$ also
configurations consisting of two nucleons and one meson. For this reason, only
two- and three-body unitarity is obeyed, and the model is suited for reactions
in the two nucleon sector only, where one pion is produced or absorbed.
Starting from a realistic pure nucleonic retarded potential, which had to be
renormalized because of the additional $\pi$ and $\Delta$ degrees of freedom, a
reasonable fit to experimental $NN$-scattering data could be achieved.
