  We investigate the lattice I(n) of clones on the ring Z_n between the clone
of polynomial functions and the clone of congruence preserving functions. The
crucial case is when n is a prime power. For a prime p, the lattice I(p) is
trivial and I(p^2) is known to be a 2-element lattice. We provide a description
of I(p^3). To achieve this result, we prove a reduction theorem, which says
that I(p^k) is isomorphic to a certain interval in the lattice of clones on
Z_p^(k-1).
