The 1-post-Newtonian approximation of perfect fluid in cosmological models
of the theory of gravitation in flat space-time is studied. The equations o
f motion are given in evolution form. At high redshifts the terms of post-N
ewtonian approximation are important in studying the development of inhomog
eneities on scales smaller than galaxies but at present time these terms on
ly take effect on very large scales.