Samples of neoplastic and nonneoplastic human urothelium were immediately frozen, incubated in Krebs' saline and then frozen, or incubated in 10-5 mol/L ouabain in Krebs' saline and then frozen. The frozen specimens were then planed in a cryoultramicrotome and examined by low-temperature scanning electron microscopy. X-Ray microanalysis was performed on the superficial urothelial cells. Neoplastic cells immediately frozen and those incubated in Krebs' saline had significantly higher K+/Na+, K+/P, and K+/Cl- ratios and lower Na+/P and Cl-/P ratios than nonneoplastic cells. Incubation in ouabain led to a fall in the K+/Na+, K+/P, and K+/Cl- ratios and a rise in the Na+/P and Na+/Cl- ratios in both neoplastic and nonneoplastic cells and effectively nullified the difference between them. These results are consistent with the concept that in neoplasia a primary event is stimulation of Na+/H+ exchange, which leads to secondary stimulation of the ouabain-sensitive Na+/K+ ATPase pump.