This work is a continuation of the anatomical study in which safe approach zones through the floor of the fourth ventricle--infrafacial and suprafacial--were morphologically and morphometrically defined (Acta Neurochir (1997) 139: 1014-1019). The purpose of cytoarchitectonic study was to analyze correlation between morphometry of the facial colliculus and hypoglossal triangle and localization of the corresponding cranial nerves nuclei in the brainstem tegmentum in order to verify morphometrical borders of the previously defined zones. Morphometrical evaluation of the fourth ventricle floor of 10 examined brainstems was initially performed. Distances from obex to the rostral portion of hypoglossal triangle and facial colliculus were determined. Then a series of axial sections of each specimen, stained for Nissl substance, were analyzed to define the distance from obex to the rostral portion of the hypoglossal and abducens nuclei. Distances of motor trigeminal and facial nuclei from the midline sagittal plane were also measured. The obtained results allowed morphometrical determination of the infra-abducental and supra-abducental region of safe entry into the brainstem tegmentum. Infra-abducental region corresponds to infrafacial safe approach zone and supra-abducental to suprafacial zone. The distance of the rostral portion of facial colliculus from obex was longer than the distance of the rostral pole of abducens nucleus from obex in every examined specimen (by 0.7 mm on average). A very similar correlation between the distance of the rostral margin of hypoglossal triangle and localization of the rostral pole of hypoglossal nucleus was found. The rostral portion of hypoglossal triangle was longer by 1.5 mm on average. The obtained results show that previously defined infrafacial and suprafacial safe approach zones via the fourth ventricle floor correspond morphometrically to tegmental regions of safe entry--infra-abducental and supra-abducental respectively. It suggests that morphometrical evaluation of the fourth ventricle floor proposed by the authors could be useful in the intra-operative determination of safe entry via the rhomboid fossa into the brainstem tegmentum.