Let M n be the algebra of all n x n matrices over a field IF, where n > 2. Let : S -*• M n such that F((A)(j>(B)) = F(AB)for various families of functions F including all the unitary similarity invariant functions on real or complex matrices. Very often, these mappings have the form A i-> iJ.(A)S(a(ajj))S~{ for all A = (a,-y) e S for some invertible S e M n , field monomorphism a of F, and an P-valued mapping \x defined on S. For real matrices, a is often the identity map; for complex matrices, a is often the identity map or the conjugation map: z i-> z. A key idea in our study is reducing the problem to the special case when F : M n ->• {0, 1} is defined by F(X) = 0, if X = 0, and F(X) = 1 otherwise. In such a case, one needs to characterize > : S -> M n such that 4>(A)4>(B) = 0 if and only if AS = 0. We show that such a map has the standard form described above on rank one matrices in S.2000 Mathematics subject classification: primary 15A04, 15A60, 15A18.