  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 r\^ole 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.

General Relativity and Quantum Cosmology;High Energy Physics - Phenomenology