We studied the microscopic morphological changes on the congenital hydrocephalus in the inbred rat, LEW/Jms in the newborn and the gestational day 20, 18 and 17 to clarify the etiology especially focusing on the aqueductal change by sagittal section. At 1 day old, hydrocephalus was not clear by outer appearance, however, it was evident by sectioning. Ventriculomegaly was limited to the lateral and third ventricles and the aqueduct was obstructed. At the gestational day 20 and 18, hydrocephalic rats showed occluded aqueducts, which were basically the same findings as the hydrocephalus in the newborn rats. At the gestational day 17, all 8 examined showed the same size in the ventricles, however, aqueductal obstruction was recognized in one of them. The other 7 rats showed the patent aqueduct. From these observation, the rat showing aqueductal obstruction at the 17 gestational day seemed to develop into the hydrocephalus by aging. So the aqueductal obstruction was considered as the primary change and not the secondary phenomenon. The site of obstruction was localized at the anterior part of the aqueduct (the level of anterior colliculus) at every stages and the thickness or hypertrophy of the tegmentum of this level was shown. Aqueductal obstruction following the developmental anomaly of the midbrain in the embryonic stage might be the primary cause for the congenital hydrocephalic rat LEW/Jms. (Shoni no Noshinkei 13: 257–262, 1988)