1. Cerebral tissues were prepared for incubation by cutting them from the brain rapidly and in situ, and the calcium concentration in the incubating medium was altered from the customary 2.8mm to 0.75mm. This provided incubated cerebral cortex with fluid and ion content more closely resembling that of the brain in vivo than hitherto obtained. 2. From a systematic difference in size between inulin spaces of slices with one and those with two cut surfaces, it was estimated that cutting directly affected a layer 0.02mm. thick. On the basis of the volume of this layer, it was calculated that the portion of the tissue not affected by cutting had an inulin space of 258 mul./g. initial wt., and during the process of preparation and incubation had gained 30muequiv. of sodium and 17 muequiv. of chloride/g. and had lost 14 muequiv. of potassium/g. 3. Several aspects of the ion content of the incubated tissue were compatible with the observed membrane potential of -60mv between cellular and extracellular phases. 4. In response to electrical stimulation, sodium of the non-inulin space increased from 28 to 57 muequiv./g., potassium decreased from 68 to 48 muequiv./g. and chloride increased from 16 to 22 muequiv./g. in the non-inulin space. These changes were complete in about 6min., and thereafter the concentrations remained steady during continued stimulation. Initial rates of change were 460 muequiv./g./hr. for sodium and 480 muequiv./g./hr. for potassium. 5. After stimulation was stopped the ionic composition of the tissue returned completely to its pre-stimulation state within 10min. Initial rates for extrusion of sodium and gain of potassium were 160 and 230 muequiv./g./hr. respectively.

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