Intracellular sodium and potassium concentrations in toad and frog oocytes during development.

1. During growth of oocytes of Bufo bufo from approximately 700-2000 mum in diameter, intracellular Na rose from 20 to 120 m-mole/l. cell water while K fell from 120 to 15 m-mole/l. cell water.2. In Rana temporaria Na similarly rose from 9 to 190 m-mole/l. cell water and K fell from 126 to 28 m-mole/l. cell water.3. At the same time in both Bufo and Rana the ouabain-sensitive Na efflux, the ouabain-insensitive Na efflux, and the total Na influx all increased.4. It is concluded that the predominant process is an increase of passive Na permeability. The increase of apparently active Na efflux may be secondary to the consequent rise in intracellular Na.