Composition of fluid obtained from choroid plexus tissue isolated in a chamber in situ.

1. A method was developed for isolating a segment of the choroid plexus of the lateral ventricle of the cat brain in a chamber in situ.2. A comparison of electrolyte and protein concentrations in serum, ultrafiltrate of serum, cisterna magna fluid, fluid accumulated in the chamber and fluid collected from the choroid plexus by another technique, demonstrates that the chamber fluid is a secretory product of the choroid plexus.3. The rate of fluid formation in the chamber was 0.4 mul. min(-1) mg(-1) of tissue, a value in good agreement with reports in the literature.4. The observation that the concentration of K(+) in choroid plexus fluid was lower than that in the serum ultrafiltrate suggests that K(+) is regulated by an active transport process at the choroid plexus.5. Significant correlation was found between electrolyte values and the protein content of the chamber fluid. This suggests that plasma is the probable source of the protein in the chamber fluid and that K(+) and probably Ca(2+) and Mg(2+) in c.s.f. are subject to active homoeostatic regulation by the choroid plexus.6. The technique described provides a new opportunity to study in detail the functional characteristics of the choroid plexus.