Patients with implanted electrodes often require an MRI exam. This exposes them to the risk of radio-frequency induced heating. In this study heating of implantable electrodes of diameters between 0.3-4mm were evaluated in a 1.5T MRI system. In situ temperature measurements were compared to simulations of the specific absorption rate to assess local and total dissipated power. Measurements showed temperature increases between 0.8-53K. Compared to large electrodes, small electrodes are subject to less dissipated power, but more localized power density. Thus, smaller electrodes might be classified as safe in current certification procedures but may be more likely to burn tissue.