  We study the nature of the see-saw mechanism in the context of renormalizable
SO(10) with Higgs fields in the 10-plets and 126-plet representations, paying
special attention to the supersymmetric case. We discuss analytically the
situation for the second and third generations of fermions ignoring any CP
violating phase. It is shown that b-tau unification and large atmospheric
mixing angle strongly disfavor the dominance of the type I see-saw.
