Experiments have been made to determine the main route by which radio-iodinated albumin reaches deep cervical lymph from cerebrospinal fluid (c.s.f.) in the anaesthetized rabbit. Other factors, influencing drainage through this pathway, have been investigated. After single injection of [125I]albumin into a lateral ventricle of control rabbits, a mean of 14.8% of the radioactivity lost from brain-c.s.f. was recovered during 6 hr in the lymph of the cannulated jugular trunk of one side. Injection of kaolin into the olfactory fossa or sealing of the cribriform plate with cyanoacrylate glue reduced the recovery of [125I]albumin to 3.3% and 1.9% respectively at 2-3 weeks after the procedure designed to block the cribriform plate. This confirms the traditional view that the major connexions between c.s.f. and deep cervical lymph is via prolongations of subarachnoid space around the olfactory nerves, leading into the interstitial spaces of the nasal submucosa. The dense lymphatic plexus in this tissue is known to drain into the retropharyngeal (deep cervical) lymph nodes. Constant infusion of artificial c.s.f. into a lateral ventricle at 10 microliters/min or 30 microliters/min, in order to approximately double or quadruple flow through the system respectively, decreased the recovery of intraventricular [125I]albumin to 8.1% and 6.9% respectively. It also appeared that the increased c.s.f. pressures induced forced relatively more radioactivity from inside the skull into the c.s.f. spaces of the spinal cord. Maintaining the rabbit prone but at 20 degrees from the horizontal caused recoveries of [125I]albumin in lymph of 17.6% (head-up position and 6.6% (head-down). The amounts of radioactivity in nose and spinal cord markedly increased and decreased respectively in the head-down position. They changed in the opposite directions in the head-up position. The amounts of [51Cr]EDTA, [125I]metrizamide and [14C]inulin in deep cervical lymph were negligible after intraventricular injection. Estimations of the ratio of [51Cr]EDTA/[125I]albumin in various tissues on the pathway into lymph, together with measurements of arterio-venous fluxes across the retropharyngeal nodes, indicate that [51Cr]EDTA passed from c.s.f./lymph into blood within both the nose and the lymph nodes.