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1. Autoradiography of frozen cells labelled with ²²Na has been used to locate a sequestered fraction of internal Na in the oocyte which exchanges very slowly or not at all with external Li.

2. Relative grain density in nucleus and cytoplasm, measured photometrically, was used as an indication of ²²Na distribution within the oocyte. In test experiments grain density fell to 50% within 19 µm of the edge of the section. Owing to the large diameter of the oocyte (> 600 µm) and its nucleus (> 200 µm), this resolution was adequate to determine cytoplasmic/nuclear (C/N) ratios of grain density.

3. In oocytes fully loaded with ²²Na, the mean C/N ratio was 0·92 ± 0·03 (n = 11). After 5 hr exchange in Li-substituted Na-free Ringer solution, the mean C/N ratio was 2·18 ± 0·04 (n = 11). After 5 hr exchange in Ringer solution as a control, the mean C/N ratio was 1·39 ± 0·18 (n = 7). The cytoplasm thus contained a fraction of ²²Na inexchangeable with Li, and more slowly exchangeable with Na than that in the nucleus.

4. The non-Li-exchangeable fraction of internal Na thus revealed appeared to be quantitatively similar to that already demonstrated by studies of ²²Na fluxes and of internal Na activity by means of Na-sensitive micro-electrodes.