Abstract: A simulator, for use with stereoradiographs, as a neurosurgical aid in locating potential paths for the implantation of probes in selected locations of the brain. It has a support, on the top surface of which is mounted a pair of radiographs disposed for stereoscopic viewing. An elongated ruler lies over the stereoradiographs and is movable in translation over them. A pair of transparent flat plotting plates are mounted on the ruler and are slidable therealong. Each plotting plate has a circular hole and a transparent circular disc is rotatable around this hole; a radial line extending from the center of the disc giving a "floating line" in the stereoscopically viewed model to simulate a probe to be implanted in the brain.