The Pelorus stereotactic system guided by magnetic resonance imaging was used to implant intracerebral depth electrodes for monitoring seizure activity. This stereotactic system is frameless and does not require the use of a computer. It is based on the concept of a ball-and-socket type stereotactic arc director and uses the center-of-arc principle to establish a trajectory for electrode placement. The system not only allows the use of the orthogonal approach, but also provides ample working space and flexibility to choose different entry points and trajectory angles.