In this work a system capable to measure the magnetoresistance (MR) effect on organic based devices was mounted and characterized. The MR effect was recently discovered on devices performed by using thin films of non-magnetic organic materials (a number of polymers and small molecules). The mounted system is able to measure small variations in the device current (up to nanoampere), at room temperature, using the magnetic field modulation technique (MFMT). In the MFMT two different magnetic fields, one DC and another AC, are applied on the region where the device is placed. The current variation of the device, due to the DC magnetic field applied, is detected by a Lock-in amplifier in phase with the AC magnetic field. In addition to the installation of the system, this work allows to study the behaviour of MR effect from the performed devices, as a function of some parameters of the system. Moreover, a preliminary investigation of the MR effect as a function of different organic compounds was also carried out. This study was divided in two parts: in the first, the organic material Alq3 was used as electron transporting and emission layer and four different organic materials were used as hole transporting layer in the devices. In the second part, the PEDOT polymer was used as hole transporting layer whereas two different rare-earth compounds were used as electron transporting and emission layer to produce the devices. Finally, an investigation of the MR dependence as a function of the degradation of the produced devices was achieved.