The Na+, K+, and inorganic phosphate levels of the plasma and sartorius muscle of the toad Bufo marinus were determined.

Soaking in normal Ringer brought about the usual cation shifts, but did not alter the level of inorganic phosphate in the cell.

Increases in the external phosphate level brought about an increase in the internal phosphate, but the apparent phosphate space of muscle is somewhat smaller than the apparent Cl- space.

Phosphate spaces were compared with inulin spaces and were found to be significantly greater.

Alteration of the H+ concentration of the high phosphate Ringer did not alter the partition of phosphate across the cell membrane.

These results have been found to be consistent with the theory of a three compartment system for muscle, wherein the tissue is assumed to consist of an extracellular phase, and two intracellular phases. The inorganic phosphate of the cell is assumed to be adsorbed onto the "ordered phase," and increments in organic phosphate found on raising the external level are assumed to take place in the "free intracellular phase."

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