Measurements of cytoplasmic and nuclear Na, K and Cl have been made by electron microprobe analysis on freeze-dried sections of oocytes of Bufo bufo, using standards of bovine plasma albumin and gamma-globulin. Concentrations were obtained per kilogram of dry mass, were converted to concentrations per litre of water content using known figures for water and solid concentration of nucleus and cytoplasm, and were then compared with measurements on cells from the same animal obtained by flame photometry. 2. In fresh oocytes concentrations were (mean +/- S.E. of mean in m-mole/l. H2O) in cytoplasm Na 10.9 +/- 1.95, K 70.2 +/- 3.22, Cl 98.8 +/- 11.0, and in nucleus Na 10.4 +/- 1.79, K 266.4 +/- 22.8, Cl 91.3 +/- 9.0. 3. After treatment with Na-free Ringer (Li substituted for Na) for 5 hr, concentrations were in cytoplasm Na 11.1 +/- 2.44, K 64.4 +/- 5.7, Cl 88.7 +/- 8.8, and in nucleus Na 2.4 +/- 0.73, K 141 +/- 13.9, Cl 75.0 +/- 6.7. Na inexchangeable with Li therefore lay in the cytoplasm but not in the nucleus as previously shown by autoradiography. 4. For K electron microscopic analysis measurements agreed well with those obtained by flame photometry but the former measured only 35% of Na measured by flame photometry. This discrepancy may be due either to technical difficulties with the electron microprobe analysis or to localization of Na in the cytoplasm.
