[en] The stationary two-wave interaction in an optically uniaxial photorefractive crystal on a transmission photorefractive grating produced due to the photogalvanic or diffusion mechanism is considered. The possibility of the nonunidirectional amplification of a weak wave, which is consistent with a change in its polarisation state caused by the interaction, is shown. The conditions of the nonunidirectional energy exchange in Fe:LiNbO₃ and SBN crystals with the local and photorefractive responses, respectively, are determined. (nonlinear optical phenomena)