  We investigate the periodic Anderson model in the presence of an external
magnetic field, using dynamical mean-field theory in combination with the
modified perturbation theory. A metamagnetic transition is observed which
exhibits a massive change in the electronic properties. These are discussed in
terms of the quasiparticle weight and densities of states. The results are
compared with the experimental results of the metamagnetic transition in
CeRu_2Si_2.
