Brain tissue damage in congenital hydrocephalic rat (LEW/Jms) was studied in the aspects of the hydrocephalic brain edema and the changes of the vascular apparatus. The characteristic findings in this study were the changes of the small blood vessels and the intracerebral cavity formation. In the acute stage of hydrocephalus (2 to 5 days after birth), spongy appearance and necrosis of the brain edema were observed in the periventricular white matter. The stenotic or obstructive vascular changes were located in connection with the hydrocephalic brain tissue. In this stage, the intracerebral cavity was formed particularly in the periventricular edematous white matter resulting in a thinning of the occipital lobes. In the late stage of hydrocephalus (9 to 15 days after birth), the lateral ventricles were severely dilated, and a markedly dilatated intracerebral cavity was observed in the periventricular white matter. The edematous area was observed adjacent to the dilated lateral ventricles or the intracerebral cavity. In the late stage, the number of small vessels filled with carbon black decreased in the area of the CSF edema when compared to the acute stage, and many obstructive blood vessels were observed in the same area. Moreover, dilatated blood vessels without carbon black were observed in the border zone between the normal and the edematous area adjacent to the intracerebral cavity. These vascular changes may occur by the accumulation of the CSF as well as the mechanical compression, and consequently lead to the microcirculatory disturbance. These microcirculatory disturbances may contribute to the intracerebral cavity formation with the accumulation of the CSF in the extracellular space.(ABSTRACT TRUNCATED AT 250 WORDS)