  Two integrable spin ladder systems with different types of impurities are
proposed. The impurities are introduced in such a way that the integrability of
the models is not violated. The models are solved exactly and the Bethe ansatz
equations as well as the energy eigenvalues are obtained. We show for both
models that a phase transition between gapped and gapless spin excitations
occurs at a critical value of the rung coupling J. In addition, the dependence
of the impurities on this phase transition is determined explicitly.
Remarkably, in one of the models a decreasing of the spin gap with increasing
impurity strength is found.
