  In this paper we discuss limits on magnetic fields that could have been
present at nucleosynthesis time. We considered several effects that could be
relevant modifing light elements relic abundances. They include: changes in
reaction rates, mass shifts due to strong and electromagnetic interactions,
variation of the expansion rate of the Universe due to both the magnetic field
energy density and the increasing of the electrons density in overcritical
magnetic fields.
  We find that the latter is the main effect. It was not taken into account in
previous calculations. The allowed field intensity at the end of
nucleosynthesis ($T =1\times 10^9\;^o{\rm K}$) is $B \le 3\times 10^{10}$
Gauss.
