  We examine the validity of the generalized second law (GSL) of gravitational
thermodynamics in the context of interacting $f(R)$ gravity. We take into
account that the boundary of the universe to be confined by the dynamical
apparent horizon in a flat FRW universe. We study the effective equation of
state, deceleration parameter and GSL in this interaction-framework. We find
that the evolution of the total entropy increases through the interaction term.
As a example, we consider a $f(R)$ gravity with a power-law dependence on the
curvature $R$. Here, we find exact solutions for a model in which the
interaction term is related to the total energy density of matter.
