This dissertation summarizes the experimental study of novel magnetoresistance (MR) properties in the GaAs/AlGaAs 2D electron systems (2DES). The first part of the dissertation discusses the mutual influence between current-induced (I_DC ) negative-giant magnetoresistance (GMR) and radiation-induced MR oscillations. The results show that MR oscillations do not modify the negative-GMR, and the MR oscillatory extrema disappear asymmetrically with increasing I_DC. The second section discusses the influence of temperature on the weak localization-like narrow negative-MR effect that appears over the interval -0.025 ≤ B ≤ 0.025 Tesla. Results suggest that the narrow negative-MR effect could be due to coherent backscattering of electrons from remote ionized donors in the GaAs/AlGaAs system. The last section analyses the influence of microwave (MW) power on the narrow negative-MR effect. It turns out that the electrons heating due to the absorption of MW radiation suppresses the narrow negative-MR effect about the null magnetic field.