We have studied the ionization of Rydberg-excited xenon atoms in THz-laser fields and by quantum dynamical calculations. The experimental threshold laser field strength for 10% ionization probability follows an n*^-1.68 (ω/2π=1.04 THz) dependence (n* effective principal quantum number) with additional weak resonance structures and shows that ionization does not occur by a Landau-Zener mechanism. At scaled frequencies of Ω = 0.71 to 5.6 the simulated threshold fields for ionization in oscillatory fields show a dependence on the principal quantum number n of n^-4.1 to n^-1.35.