Operating resonant mass detectors set interesting bounds on diffused backgrounds of gravitational radiation and in the next five years the wide-band interferometers will also look for stochastic sources. In this lecture the interplay among relic GW backgrounds and large scale magnetic fields will be discussed. Magnetic fields may significantly affect the thermal history of the Universe in particular at the epoch electroweak symmetry breaking and shortly after. A review of some old an new results on the spectral properties of stochastic GW backgrounds will be presented. The possible rôle of primordial magnetic fields as a source of gravitational radiation will be outlined. It will be shown that the usual bound on stochastic GW backgrounds coming from the standard big bang nucleosynthesis (BBN) scenario can be significantly relaxed.