The second-order nonlinear optical properties of purple membrane-poly(vinyl alcohol) films (PM-PVA) are investigated by second-harmonic generation. The magnitudes and the phases of the second-order molecular polarizabilities of the bacteriorhodopsin molecule in the bR570 and M412 states are obtained. Comparing these results with those obtained for free retinal and retinal Schiff bases spread at air-water interfaces indicates that, upon electronic excitation, the dipole moment change of the retinylidene chromophore in bR570 appears larger than for the chromophore when it is not complexed to the protein. The observed second-harmonic signal from a PM-PVA film bleached by a CW light source agrees well with the square of the calculated mole fraction of the bR570 intermediate. The implications of our findings are discussed.