Twenty rabbits were intrathecally perfused with horseradish peroxidase (HRP) without increasing the cerebrospinal fluid (CSF) pressure. Histologic and ultrastructural studies of the olfactory nerve and nasal mucosa showed HRP along the space between peri- and epineurium down to the smaller nerve branches in the mucosa. Within the mucosa, HRP leaked out from the epineurium and flowed into: (a) lymphatic vessels; (b) mucous glands and ducts; (c) intercellular spaces of the nasal pseudostratified nasal epithelium; and (d) venules. These findings could account for "bulk" flow CSF absorption into the lymphatic system, the nasal cavity, and to a lesser degree, the venous system; also explain CSF rhinorrhea in man and experimental animals; as well as lymphatic drainage of CSF into cervical lymph nodes.