  A self-consistent relativistic two-fluid model is proposed for
one-dimensional electron-ion plasma dynamics. A multiple scales perturbation
technique is employed, leading to an evolution equation for the wave envelope,
in the form of a nonlinear Schr\"odinger type equation (NLSE). The inclusion of
relativistic effects is shown to introduce density-dependent factors, not
present in the non-relativistic case - in the conditions for modulational
instability. The role of relativistic effects on the linear dispersion laws and
on envelope soliton solutions of the NLSE is discussed.
