The external shunt operation on the endolymphatic sac is based on the concept that it will drain excess endolymph and thus arrest the progression of endolymphatic hydrops. We performed histological studies on 46 temporal bones from 13 cases of bilateral and 20 cases of unilateral Meniere's disease to evaluate the status of the pathways of longitudinal flow of endolymph to the sac. The endolymphatic ducts were blocked in 8 specimens (17%), the endolymphatic sinuses in 9 (19.5%), the utricular ducts in 12 (26%), the saccular ducts in 7 (15%), and the ductus reuniens in 27 (59%). These blockages arrested longitudinal flow from both the pars superior and inferior in 21 cases (46%), the pars superior only in 3 (6.5%), and the pars inferior only in 16 (35%). In the 6 ears with all pathways open, 2 were found to have fistulae between the saccules and the perilymphatic spaces, which theoretically results in internal shunting, thus alleviating the need for the external shunt procedure. In the aggregate, therefore, 42 (91%) of 46 ears showed either areas of blockage of longitudinal flow or internal shunts that would theoretically negate the value of external endolymphatic shunt procedures. These temporal bones, however, are from an autopsy population and the severity of pathological changes is probably greater than it would have been at an earlier age when external shunt surgery might have been a therapeutic consideration.