  We perform three-dimensional numerical simulations of relativistic (with a
Lorentz factor of 10), non magnetized jets propagating in a uniform density
environment, in order to study the effect of the entrainment and the consequent
deceleration. Our simulations investigate the jet propagation inside the galaxy
core, where, most likely, the deceleration occurs more efficiently. We compare
cases with different density and pressure ratios with respect to the ambient
medium finding that low density jets are efficiently decelerated and reach a
quasi steady state in which, over a length of 600 jet radii, slow down from
highly to sub-relativistic velocities. At the opposite, denser jets keep highly
relativistic velocity over the same length. We discuss these results in
relation to the Faranoff Riley (FR) radio-sources classification. We infer that
lower density jets can give rise to FR 0 and FR I radio-sources, while higher
density jets may be connected to FR II radio-sources.
