  We suggest an explanation of the "Pioneer effect" based on the interaction of
the spacecraft with a long-range scalar field, $\phi$. The scalar field under
consideration is external to gravity, coupled to the ordinary matter and
undergoes obedience to the weak equivalence principle. In the weak fields limit
it result a long-range acceleration $a_{P}$, asymptotically constant within the
region of the solar system hitherto crossed by the spacecraft.
