  We discuss the microscopical justification of dissipation in the model
nonrelativistic Fermi and Bose systems with weak local interactions above phase
transitions. The dynamics of equilibrium fluctuations are considered in Keldysh
- Schwinger framework. We show that the dissipation is related to pinch
singularities of the diagram technique. Using Dyson - Schwinger equation and
the two-loop approximation we define and calculate the attenuation parameter
which is related to exponentiality of Green's functions decay. We show that the
attenuation parameter is the microscopic analogue of the Onsager kinetic
coefficient and it is related to attenuation in the excitation spectrum.
