An investigation has been made of the effects of electromagnetic fields of angular frequency 2 on the I-V characteristics of superconducting Pb films with quantized resistances. The effects of both cw (10 invsec<<109 invsec) and pulsed (109 invsec<<1010 invsec) fields on the different current components of the films have been studied. It is found that the low-frequency response (108 sec-1) is qualitatively different from that of higher frequencies. The low-frequency response can be understood in terms of the time-dependent momentum of the electron pairs which leads to an ac current and also a time-dependent variation of the energy gap. At higher frequencies the experimental results indicate that the time average of the energy gap is strongly power dependent. The observed behavior at high frequencies is consistent with theoretical predictions based on the time-dependent Ginzburg-Landau equations. © 1977 The American Physical Society.