Solution conformations and dynamic mobilities of novel amphoteric copolymers were studied for aqueous solutions of p(sodium 2-methacryloyloxyethanesulfonate-co-2-methacryloy- loxyethyltrimethylammonium iodide) [P(NaMES-co-METMAI)] as a function of composition, concentration, and addition of HCl or KCl. As HCl or KCl was added successively to 5 w/v% copolymer solutions, the reduced viscosities of the solutions were reduced except the 1 : 1 copolymer solution. The dynamic mobility of the cationic side chain was estimated with the quadrupolar N-14-NMR relaxation technique. The motion of the interacting counterion, Na+, was studied by Poisson-Boltzmann electrostatic theories and the quadrupolar Na-23-NMR. Since the intramolecular 1 : 1 ion pair between MES(-) and METMA(+) acts as an effective charge-screening factor, the copolymers P(NaMES-co-METMAI) behave as ordinary polyelectrolytes with the exception of the 1 : 1 copolymer.