We report a set of experimental data on third-harmonic generation at m
icrowave frequencies in YBa2Cu3O7 single crystals near T(c) at zero ap
plied field. A theory is also developed which fully accounts for the e
xperimental data. The theory is based on the two-fluid model and the h
ypothesis that the microwave magnetic field modulates the partial conc
entrations of both normal and superconducting electrons. l