We studied the microscopic morphological changes in congenital hydrocephalus in the inbred rat, LEW/Jms, on gestational days 17, 18, and 20 and during the neonatal period to clarify the etiopathogenesis, focusing particularly on the aqueductal changes. At 1 day of age, ventriculomegaly was limited to the lateral and III ventricles, and the aqueduct was obstructed, with the appearance of simple stenosis. On gestational days 20 and 18, the hydrocephalic rats showed occluded aqueducts, which paralleled the finding of hydrocephalus in the newborn rats. On gestational day 17, all eight models examined showed the same size ventricles. However, an aqueductal obstruction was observed in one of them. The other seven rats showed the aqueduct patent. From these observations, it can be concluded that the rat shows an aqueductal obstruction on gestational day 17 and appears to develop hydrocephalus with age. The aqueductal obstruction was considered to be the primary change and not a secondary phenomenon. The site of obstruction was the anterior part of the aqueduct (level of anterior colliculus) at every stage. Aqueductal obstruction following the developmental anomaly of the midbrain in the embryonic stage might thus be the primary cause of congenital hydrocephalus in rats LEW/Jms.