An anatomical study of the normal anterior inferior part and the floor of the third ventricle has been made and its radiological appearance shown. Specimens and casts were analyzed and correlated with ventriculograms using watersoluble contrast media. The detailed appearance of the optic recess and sulcus of Monro, which has not been well described in the neuroradiological literature, is illustrated and detailed. The changes in the optic recess in hydrocephalus are discussed. Deformities of the different segments of the floor as a result of tumors of the posterior fossa and hydrocephalus are shown.