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1. The activity of potassium, a_K, in the cytoplasm of oocytes from the toad, Bufo bufo, as measured by potassium-sensitive glass micro-electrodes, was 82 mM. The concentration of potassium, C_K, in oocytes from the same ovaries, as determined by flame photometric analysis, was 113 mM. The ratio a_K/C_K = 0·73 does not differ significantly from the measured activity coefficient of the normal Ringer bathing solution, which is 0·75.

2. The activity of sodium, a_Na, in the cytoplasm of toad oocytes, as measured by sodium-sensitive glass micro-electrodes, was 9·3 mM. The concentration of sodium, C_Na, in oocytes from the same ovaries, as determined by flame photometric analysis, was 25·8 mM. The value of the a_Na/C_Na ratio in the cells, 0·36, is only about half the value of either the a_K/C_K ratio in the cells or the activity coefficient of sodium in the normal Ringer bathing solution. This implies that about half the sodium in the cell is sequestered in some manner, such that it is unavailable to affect a cation-sensitive micro-electrode.

3. When the oocytes were bathed for 5 hr in a sodium-free, lithium-substituted, Ringer solution the a_Na/C_Na ratio decreased to 0·06-0·10. This drop in the a_Na/C_Na ratio implies that the sodium available to the cation-sensitive micro-electrode can leave the cell much faster than the sequestered sodium.