Objective To investigate the effect of amiloride pre-incubation on the intracellular electrolyte levels in neoplastic and non-neoplastic urothelium. Materials and methods Samples of papillary transitional cell carcinomas (grades 1 and 2) and non-neoplastic human urothelium were either frozen immediately or after pre-incubation with Krebs saline alone or with Krebs saline containing amiloride. The frozen samples were planed in a cryo-ultramicrotome and then examined by low temperature scanning electron microscopy. Intracellular electrolyte levels were measured using X-ray microanalysis. Results In the cells frozen immediately the ratios K+/Na+, K+/P and K+/Cl- were significantly increased in neoplastic compared with non-neoplastic urothelial cells (P<0.001). This difference was also found in the cells pre-incubated with Krebs saline alone but was not present in the cells pre-incubated with amiloride. Conclusions These results were consistent with the concept that activation of the amiloride-sensitive Na+/H + membrane exchanger is a primary event in neoplasia.