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1. The objective was to discover whether the extraction of sugars and iodide from the perfused cerebral ventricles is Na⁺-dependent.

2. In the ventriculo-aqueductal and ventriculo-cisternal perfusion systems in the rabbit the extraction of ¹⁴C-labelled D-hexoses (glucose, 3-O-methyl-glucose, -methyl-glucoside and galactose), ¹³¹I^- and ²⁴Na was inhibited when 82% of the Na⁺ in the perfusion fluid was replaced by choline. The extraction returned to control levels when the Na⁺ concentration in the perfusion fluid was returned to normal.

3. Ouabain, 5 x 10^-5 M in the perfusion fluid inhibited the extraction of the above ¹⁴C sugars and ¹³¹I^-, but hardly affected that of [³H]2-deoxy-D-glucose. It enhanced the extraction of ²⁴Na. C.s.f. production was usually totally inhibited.

4. The extraction of [¹⁴C]urea remained unchanged during perfusion with low Na⁺ fluid or ouabain.

5. Recovery from brain of [¹⁴C]3-O-methyl-glucose, [³H]2-deoxy-glucose and ¹³¹I^- was low while recovery of [¹⁴C]-methyl-glucoside and ²⁴Na was high. On an equal weight basis recovery of [¹⁴C]3-O-methyl-glucose was about twelve times higher from the choroid plexus than from the brain.

6. Part of the movement of ¹⁴C sugars may be explained on basis of a Na⁺-gradient hypothesis with involvement of the Na⁺ pump at the blood—c.s.f. or blood—brain barriers.

7. The rate of c.s.f. production from the first three ventricles comprised about 40% of the rate from all four ventricles. The extraction of sugars, urea and cations was similar in both perfusion systems while the extraction of ¹³¹I^- was higher in the ventriculo-cisternal system than in the ventriculo-aqueductal system.