The fluorescence intensity (FI) on immunohistochemical staining with a monoclonal antibody (Mab 171 B5) against a synaptic vesicle-associated protein (SVP-38) was measured in the CA1 sector of the hippocampus of brains obtained from hydrocephalic and non-hydrocephalic HTX-rats aged 1, 7, 14, 21 and 28 days. FI was significantly increased on the 21st and 28th postnatal days in hydrocephalic rats as compared to in non-hydrocephalic rats, despite the presence of marked morphological stretching and rotation of the dendritic arborization of Golgi-impregnated neurons in the CA1 sector. The increase in FI appeared to indicate an increase in synaptic density. As previous reports have indicated a marked reduction in SVP-38 FI in the thin cerebral cortex of 28th postnatal day hydrocephalic brains, our findings may suggest that hippocampal synaptogenesis is more resilient than that of the cerebral cortex in hydrocephalic brains.