A method is described for the quantitative determination of free and bound solute concentrations in the cytoplasm of intact cells. The method includes (a) introduction of a gelatin gel reference phase (RP) into the cytoplasm; (b) diffusion of dissolved substances between cytoplasm and RP, (c) cell quenching to - 196 degrees C to prevent subsequent solute redistributions, (d) ultra-low temperature microdissection to isolate RP and cytoplasm samples, and (e) analysis of isolates for solute and water content. In normal oocytes of the salamander, Desmognathus ochrophaeus, free or RP Na+ and K+ are 21.0 +/- 1.1 and 128.8 +/- 2.4 mu eq/ml, respectively, and vary stoichiometrically in altered oocytes. Overall cytoplasmic concentrations are 75.2 +/- 2.7 mu eq Na+/ml and 88.6 +/- 1.5 mu eq K+/ml. Cytoplasmic chemical activities are 16.2 mu eq Na+/ml and 99.2 mu eq K+/ml, corresponding to activity coefficients of 0.22 and 1.12, respectively. The results demonstrate unambiguously that (a) oocytes actively transport Na+ and K+, and (b) cytoplasm has important binding properties which differentiate it from an ordinary aqueous solution. These cytoplasmic properties are investigated in the following paper.