A method is described for the determination of the electrophoretic mobility of single, isolated, intact, giant axons of squid and lobster. In normal physiological solutions, the surface of hydrodynamic shear of these axons is negatively charged. The lower limit of the estimated surface charge density is -1.9 x 10(-8) coul cm(-2) for squid axons, -4.2 x 10(-8) coul cm(-2) for lobster axons. The electrophoretic mobility of squid axons decreases greatly when the applied transaxial electric field is made sufficiently intense; action potential propagation is blocked irreversibly by transaxial electric fields of the same intensity. The squid axon recovers its mobility hours later and is then less affected by transaxial fields. Eventually, a state is reached in which the transaxial field irreversibly reverses the sign of the surface charge. In contrast, there is no obvious effect of electric field on the mobility of lobster axons. The mobility of lobster axons becomes undetectable in the presence of Th(4+) at a concentration which blocks the action potential, and in the presence of La(3+) at a concentration which does not affect propagation. Quinine does not alter lobster axon mobility at a concentration which blocks action potential conduction. Replacement of extracellular Na(+) by K(+) is without effect upon lobster axon mobility. The electrophysiological implications of the results are discussed.