We discuss the spectrum of very high Rydberg states as detected via ionization in weak external electric fields. For the conditions of interest, namely, states just below the ionization continuum and weak fields, the classical barrier to dissociation is extremely far out from the core. About the saddle point the potential is very shallow. It is concluded that ionization by tunneling is far too slow. Only electrons whose energy is above the classical barrier can be detected via ionization. However, not all electrons which energetically can ionize will necessarily do so. Electrons may fail to ionize if the fraction of their energy which is in the direction perpendicular to the field is high. The computed fraction of electrons which fails to ionize does depend, in a sensitive way, on the diabatic vs adiabatic switching on of the external field. More experiments and theoretical work is needed on this point. A classical procedure based on the adiabatic invariance of the volume in phase space is developed for the computation of the fraction of electrons that can surmount the classical barrier for a given field. Analytically exact results are obtained for adiabatic switching and for the sudden limit where the rise time of the field is shorter than the period of the orbit. For the case of diabatic switching (which is appropriate for very high n values), the exact classical computations on the yield of ionization show that the onset of ionization is at an energy of 4.25 F1/2 cm-1 below the ionization potential and the 50% point it as 3.7 F1/2 cm-1 for a field F in V/cm.