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Abstract

1. A new technique is presented for determining the volume of extracellular space in bowfin (Amia calva) brain during in vitro incubation. It consists of solving simultaneous equations which are applied to determine the volume of extracellular space as well as intracellular marker concentration. This technique allows for a better insight into the redistribution of marker between incubation medium and extracellular space as well as between extracellular and intracellular space.

2. Na⁺, K⁺ and Cl^- equilibrated within 10-15 min between incubation medium and extracellular space. There was no evidence of a homoeostatic mechanism controlling the concentration of these ions in the extracellular fluid, which appeared to be in equilibrium with cerebrospinal fluid. The extracellular spaces of these ions were identical: Na⁺, 23·4; K⁺, 23·3 and Cl^-, 23·2%.

3. Sorbitol equilibrated with the extracellular fluid within 45 min and indicated an extracellular space of 22·6%, nearly identical with that for electrolytes.

4. Vastly different `spaces' were obtained for [<sup>3</sup>H]methoxy inulin, which equilibrated within 45 min with a 13% space and [<sup>14</sup>C]carboxyl inulin, which showed a 46% space value for only 30 min. The difference may be explained by marker decomposition. The 9% difference between the [<sup>3</sup>H]methoxy inulin and sorbitol spaces may be explained by a `packing' factor attributable to molecular size.