Infrared and tryptophan fluorescence spectra of practically all sufficiently stable functional complexes of a highly purified preparation of membrane-bound (Na+, K+)-dependent ATPase have been measured. The formation of any functional complex was not accompanied by any considerable change of either shape or position of the tryptophan fluorescence spectrum. Only in the presence of adenine nucleotides was there a small decrease of fluorescence intensity (by 5-8%), which apparently results from a change of the sample light scattering. Analysis of the results obtained leads to the conclusion that the environment of no more than one or a few tryptophan residues may differ in all the (Na+, K+)-ATPase complexes studies. A comparison of infrared protein spectra in the region of amide I band showed that at any wavenumber the differences between them did not exceed 3% of the maximum absorption. This means that no more than 3% of protein peptide groups can change their conformation upon transition between the enzyme functional states. These results, obtained by two independent techniques, allow us to conclude that even if changes of the internal protein structure occur during the working cycle of this transport system, if they have an extremely local character.