We report the first measurements of microwave excitation and ionization of excited hydrogen atoms for scaled frequencies n03 up to 2.8. Classical 3D calculations which directly model this 36.021-GHz experiment agree quite well for n03<1, agree less well for 1n032, and do not agree for n03>2. This supports theoretical predictions that as n03 rises above 1, quantal ionization threshold fields rise above those for the onset of classical chaos; however, the data continue to reveal local stability near certain rational frequency ratios that recalls classical behavior. © 1988 The American Physical Society.