Let G be a finite group. The Bogomolov multiplier B 0 (G) is constructed as an obstruction to the rationality of (V ) G where G → GL(V ) is a faithful representation over . We prove that, for any finite groups G 1 and G 2 ,For any integer n, we show that there are non-direct-product p-groups G 1 and G 2 such that B 0 (G 1 ) and B 0 (G 2 ) contain subgroups isomorphic to ( /p ) n and /p n respectively. On the other hand, if k is an infinite field and G = N ⋊ G 0 where N is an abelian normal subgroup of exponent e satisfying that ζ e ∈ k, we will prove that, if k(G 0 ) is retract k-rational, then k(G) is also retract k-rational provided that certain "local" conditions are satisfied; this result generalizes two previous results of Saltman and Jambor [Ja]. §1. Introduction Let k be a field, and L be a finitely generated field extension of k. L is called k-rational (or rational over k) if L is purely transcendental over k, i.e. L is isomorphic