  Modification of gravity due to the curvature dependent term in the
gravitational baryogenesis scenario is considered. It is shown that this term
leads to the fourth order differential equation of motion for the curvature
scalar instead of the algebraic one of General Relativity (GR). The fourth
order gravitational equations are generically unstable with respect to small
perturbations. Non-linear in curvature terms may stabilize the solution but the
magnitude of the stabilized curvature scalar would be much larger than that
dictated by GR, so the standard cosmology would be strongly distorted.
