The mechanical compression of optic nerves, of the chiasma of optic pathways by vessels of Willis circle was revealed in 12 of 34 necropsies of extensive craniopharyngiomas and pituitary adenomas and in three of 109 patients operated on account of these tumours. The localization of pressure changes, from shallow grooves on the surface of optic structures to almost complete interruption of their anatomical continuity, depended on the localization of the tumour in relation to the chiasma. This relation was influenced in particular by two factors, the site of the primary development of the tumour and the premorbid localization of the chiasma. Impaired optic functions were the result of the action of pressure of the tumour exerted on the area and pressure of firm vascular structures on a defined portion of the optic pathway, which begins to act only when the tumour is large and has a more malignant effect than the pressure of the tumour itself.