Newt oocytes were dissected under liquid paraffin and known volumes of clean nucleoplasm and cytoplasm were obtained from oocytes in different stages of oogenesis. The samples were digested in redistilled nitric acid, diluted with deionized water, and their Na and K contents were measured by flame spectrophotometry. The results were expressed as micro-equivalents of Na and K per millilitre of nucleoplasm or cytoplasm. In oocytes of 0.3-0.5 mm diameter nucleoplasm and cytoplasm have similar Na and K concentrations, and the molar K:Na ratio is about 3:1. As the oocyte grows to maturity the nucleoplasmic Na and K concentrations do not change, but the cytoplasmic K concentration falls steadily until, in nearly mature oocytes, the cytoplasmic K:Na ratio is near 1:1. Measurements of Na and K concentrations in yolky and clear cytoplasm show that both fractions have the same K content and the concentration of K decreases in both fractions as the oocyte grows. The significance of these results is discussed in terms of the possible effects of changes in the intracellular ion balance on the morphology and synthetic activity of the chromosomes.