  A new framework for exploiting information about the renormalization group
(RG) behavior of gravity in a dynamical context is discussed. The
Einstein-Hilbert action is RG-improved by replacing Newton's constant and the
cosmological constant by scalar functions in the corresponding Lagrangian
density. The position dependence of $G$ and $\Lambda$ is governed by a RG
equation together with an appropriate identification of RG scales with points
in spacetime. The dynamics of the fields $G$ and $\Lambda$ does not admit a
Lagrangian description in general. Within the Lagrangian formalism for the
gravitational field they have the status of externally prescribed
``background'' fields. The metric satisfies an effective Einstein equation
similar to that of Brans-Dicke theory. Its consistency imposes severe
constraints on allowed backgrounds. In the new RG-framework, $G$ and $\Lambda$
carry energy and momentum. It is tested in the setting of homogeneous-isotropic
cosmology and is compared to alternative approaches where the fields $G$ and
$\Lambda$ do not carry gravitating 4-momentum. The fixed point regime of the
underlying RG flow is studied in detail.
