An attempt was made to reveal the mode of action of protons and salts on the recently discovered GTP gamma S-dependent interaction of bovine retinal rod outer segments (ROS)1 nucleoside diphosphate kinase (NDP kinase) with the complex between bleached visual receptor rhodopsin and retinal G-protein transducin in bovine ROS membranes. The properties of recombinant rat NDP kinase alpha, that is immunologically similar to the soluble NDP kinase from bovine ROS preparation, have been studied in solution by means of protein fluorescence at different pH and salt concentrations and results were compared with pH and salt effects on the binding of NDP kinase alpha to bleached bovine ROS membranes. The results suggest that NDP kinase alpha itself may serve as a target for protons and salts and mediates their effects on the interaction between the enzyme and ROS membranes.