We tested the effects of epidermal growth factor (EGF) on Na+/H+ exchanger (NHE) activity using urogastrone for treatment of Wistar rats and rat kidney tissue slices. NHE activity was monitored in isolated kidney brush border membrane vesicles by following fluorescence quenching of acridine orange. A significant increase of NHE activity was detected as early as 5 min after addition of urogastrone to rat kidney slices in vitro. In Wistar rats treated with urogastrone we also found increased NHE activity (by about 12 %). Both changes of NHE activity were the result of a significant rise of V(max) value and an apparent decrease in K(m) value in in vitro experiments. The rise of NHE activity caused by urogastrone was sensitive to the inhibitors of transcription and translation. The presence of phosphatase inhibitor, NaF, elevated NHE activity of non-stimulated as well as of urogastrone-stimulated exchanger, suggesting that phosphorylation plays an important role in Na+/H+ exchange. Osmolarity of the medium seems to regulate NHE activity in such a manner that both hyper- and hypoosmolar conditions inhibited NHE activity. The absence of Ca2+ ions produced a 60 % decrease of NHE activity. The chemical modification of histidine residues with diethyl pyrocarbonate or SH groups with N-ethylmaleimide inhibited NHE activity.