  The deviation of primordial Helium production due to a variation on the
difference between the rest masses of the nucleons is presented. It is found an
upper bound $\delta (M_{_n} - M_{_p}) \alt 0.129$ MeV, between the present and
nucleosynthesis epochs. This bound is used to analyze Wesson's theory of
gravitation; as a result, it is ruled out by observation.
