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1. Experiments were performed to determine the relative contribution of the blood plasma and of the cerebrospinal fluid (c.s.f.) to the ionic environment of neurones and glial cells within the brain of the amphibian Necturus maculosus.

2. The concentrations in the blood plasma of untreated control animals were 99 ± 2 mM for Na⁺ and 2·0 ± 0·1 mM for K⁺. In the c.s.f. the corresponding values were 112 ± 2 mM-Na⁺ and 1·9 ± 0·1 mM-K⁺.

3. By keeping animals in K⁺-rich water it was possible to raise chronically the concentrations of K⁺ in the blood plasma up to almost 5 times the normal value, close to 9 mM, while the c.s.f. concentration of K⁺ was only doubled, to about 4 mM. This behaviour of Necturus, tending to keep the K⁺ in the c.s.f. low, resembles that of mammals.

4. The membrane potential of glial cells in the optic nerve can be used as an accurate indicator for determining the K⁺ concentration in the intercellular spaces. Such determinations were made in vivo, and it was shown that the glial cells adjust their membrane potential to the changes of K⁺ concentrations in the c.s.f. and not to those of the blood plasma. In contrast, the membrane potential of skeletal muscle fibres changes according to the K⁺ concentration in the blood plasma.

5. It is concluded that the cells within the optic nerve are surrounded by an ionic environment which corresponds to that of the c.s.f. and not to that of the blood plasma. The intercellular spaces are open and ions diffuse freely into them from the c.s.f. A homeostatic mechanism operates, keeping the ion concentrations around neurones and glia within a narrow range and relatively independent of large changes in the blood plasma. This may provide relative stability for the signalling system. Similarities between the optic nerve and other parts of the central nervous system in respect to their relation to c.s.f. and blood are discussed. It seems likely that the mechanisms which control the electrolyte concentrations are similar in Necturus and in mammals.