Significantly reduced activities of glutamine synthetase (GS), which is predominantly present in glial cells, occur in the early stage of congenital hydrocephalic rat (LEW-HYR) brain development. GS activity is reported to be related to brain dysfunction. The effect of ventriculoperitoneal (VP) shunt on the suppression of GS activity was studied in the LEW-HYR. VP shunting improved the attenuation of GS activity in the LEW-HYR and the response of GS activity to methionine sulfoximine (a competitive GS inhibitor) treatment was similar to that seen in normal siblings. However, no enhancement of GS activity by hydrocortisone could be detected, although this enhancement occurs in the normal siblings. These results suggest that VP shunting is not completely effective in improving the suppression of brain GS activity in the LEW-HYR, since the suppression of GS activity in the LEW-HYR might be programmed genetically.