  A microscopic theory of zero wavelength (q=0) interaction in finite--size
systems is proposed. Its exact solution interpolates between the Coulomb
blockade and the perturbative Altshuler--Aronov theory, in the strong and weak
interaction limits respectively. The tunneling density of states and the
quasiparticle life--time are calculated. The physical nature of the q=0
component of the interaction is discussed.
