  M. Kontsevich proposed a topological construction for an invariant Z of
rational homology 3-spheres using configuration space integrals. G. Kuperberg
and D. Thurston proved that Z is a universal real finite type invariant for
integral homology spheres in the sense of Ohtsuki, Habiro and Goussarov.
  We discuss the behaviour of Z under rational homology handlebodies
replacements. The explicit formulae that we present generalize a sum formula
obtained by the author for the Casson-Walker invariant in 1994. They allow us
to identify the degree one term of Z with the Walker invariant for rational
homology spheres.
