  We construct metric perturbations of two families of isotropic expanding
universes describing gravitational waves propagating through these universes.
The waves are non--planar and owe their wave front expansion solely to the
expansion of the universes. The presence of this radiation leads to a small
perturbation of the perfect fluid matter content of the universes by the
appearance of an anisotropic stress. We then construct exact models of gravity
waves in these universes. In this case the matter content of the models
consists of the perfect fluid matter supplemented by anisotropic stress and
lightlike matter traveling with the waves. Under appropriate conditions of
approximation the lightlike matter can be neglected and the exact models
coincide with the perturbative models.
