A stereotactic instrument for precise insertion of an electrode in the brain for treatment of certain nervous system disorders, such as Parkinson's disease. A rectangular frame surrounds the head and is supported by four pins pressed against the skull. The front and back segments of the frame support radio-opaque vertical markers for alignment with the midline of the brain. The left and right segments each support a pair of adjustable radio-opaque markers for alignment with the anterior and posterior commissures of the brain. Adjustment is by turning knobs connected to a rod and gear system. Bridges between anterior and posterior markers parallel a reference line between the commissures of the brain's third ventricle. Adjustable brackets on the bridge support an arc bearing an electrode carrier.