In Alzheimer's disease, the neuritic or senile amyloid plaques in hippocampus and association cortex, the diffuse plaques in brain areas such as the cerebellum and sensorimotor cortex, and the amyloid deposits in the walls of pial and parenchymal blood vessels are mainly composed of amyloid beta-peptides. In the present study, either soluble 40-residue amyloid beta-peptide radiolabeled with 125I (I-sAbeta) or [14C]polyethylene glycol ([14C]PEG, a reference material) was briefly infused into one lateral ventricle of normal rats. By 3.5 min, 30% of the I-sAbeta was cleared from ventricular CSF into blood; another 30% was removed over the next 6.5 min. No [14C]PEG was lost from the CSF-brain system during the first 5 min, and only 20% was cleared by 10 min. Much of the I-sAbeta that reached the subarachnoid space was retained by pial arteries and arterioles. Virtually no I-sAbeta was found in brain. The clearance of amyloid beta-peptides from the CSF-brain system, reported herein for normal rats, may be reduced in Alzheimer's disease, thus contributing to amyloid deposition in cerebral tissue and blood vessels.