Hydrocephalic and nonhydrocephalic HTX rats served, respectively, as experimental and sham operation animals. They were treated by insertion of ventriculoperitoneal (V-P) shunts at 7 days after birth (early shunt). Monoclonal antibodies against synaptic vesicle proteins (SVP-38) and developmentally regulated brain proteins (drebrins) were used to assess these two synapse-related proteins by means of either a quantitative immunohistochemical method or a qualitative immunoblot analysis. The amount of SVP-38 progressively increased during a 3-week period after birth in both hydrocephalic and nonhydrocephalic HTX rats, but decayed suddenly at 4 weeks after birth in hydrocephalic HTX rats. When the hydrocephalic HTX rats were treated with V-P shunts, such perturbations in postnatal changes of SVP-38 were prevented completely. In nonhydrocephalic HTX rats, only the embryonic form of drebrins was detected at 1 day after birth, and this disappeared at the end of the 3rd week. The adult form of drebrin could be detected at 1 week after birth, and this completely replaced the embryonic form by 3 weeks of age. However, the postnatal decay of the embryonic form was considerably delayed in hydrocephalic HTX rats. These observations indicate that since synaptogenesis in the brain of hydrocephalic HTX rats has already been disturbed at the prenatal periods, which is the initial process of hydrocephalus, early shunt treatment of hydrocephalus would be beneficial not only for repairing but also for preventing impaired synaptogenesis.