Abstract: This invention relates to an adapter for definition of the position of brain structures on a patient's head by means of spatial coordinates in computerized tomography and NMR examination and transferring the coordinates to a stereotactic apparatus, said adapter including ear supports carried by a first arm and a further support for reproducible fixation of the adapter to the patient's head, with two side frame parts adapted to be fitted on either side of the patient's head.The novelty of this invention is that the adapter includes a second arm extending angularly to said first arm and forming together with the first arm and a further, third arm a triangular side frame structure, the side frame parts on either side of the patient's head being interconnected by means of connecting means which carry the further support and a laterality indicator means consisting of an anterior laterality indicator and two posterior laterality indicators for definition of a reference plane through the midline of the adapter.