Arachnoid granulations are not always present in lower mammals and primate newborns. In order to visualize the route for the cerebrospinal fluid (CSF) to drain into the venous system, horseradish peroxidase (HRP) was injected into the lateral ventricle or cisterna cerebellomedullaris of the mouse and rat. From 30 to 60 min after the commencing of a slow infusion for 15-30 min of 0.05-0.1 ml solution containing 10-20 mg HRP, the mouse, whose skull had been exposed, was dropped into cold acetone at dry ice temperature; other animals were fixed by perfusion with aldehyde solution. The frozen head was dissected in a cryostat kept at -18 degrees C to remove the skull, but leave the dura mater and the falx cerebri. The brain with meninges was cut into 30-45 microns sagittal sections in the cryostat, and processed for peroxidase reaction. The perfusion-fixed brains were used for scanning electron microscopy and for electron microscope observation of the tracer. The reaction product was found within fenestrated venous capillaries of the choroid plexus. The route for the HRP in the CSF to drain into the sinus rectus via the vena choroidea and vena cerebri magna was directly visualized in the mouse.