The electrolyte composition of toad urinary bladder epithelial cells has been measured using the technique of electron microprobe analysis. Portions of hemibladders, which had been mounted in chambers and bathed with a variety of media, were layered with albumin solution on their mucosal surfaces and immediately shock-frozen in liquid propane at -180 degrees C. From the frozen material 1--2 micrometer thick cryosections were cut and promptly freeze-dried for 12 hr at-80 degrees C and 10(-6) Torr. Electron microprobe analysis using a scanning electron microscope, an energy dispersive X-ray detector, and a computer programme, to distinguish between characteristic and uncharacteristic radiations, allowed quantification of cellular ionic concentrations per kg tissue wet wt by comparison of the intensities of the emitted radiations from the cells and from the albumin layer. Granular, mitochondrial-rich, and basal cells, and the basal portions of goblet cells, showed a similar composition, being high in K (about 110 mM/kg wet wt) and low in Na (about 13 mM/kg wet wt). The apical portions of goblet cells were higher in Ca and S and lower in P and K, presumably reflecting the composition of the mucus within them. With Na-Ringer's as the mucosal medium, cells gained Na and lost K, when their serosal surfaces were exposed to ouabain, 10(-2) M. Replacement of mucosal Na by choline virtually prevented these ouabain-induced changes. Cellular ion contents were unchanged when Na in the serosal medium was replaced by choline. No differences in Na and K concentrations were detected between nuclei and cytoplasm. These results provide independent support for the hypothesis the the cellular Na transport pool in toad bladder epithelial cells derives exclusively from the mucosal medium and that no important recycling of Na occurs from the serosal medium to the cells.