Net uptake of Cl(-) into root tips of mung bean (Phaseolus aureus) increases steadily with increasing external concentrations from 1 to 60 mm. Membrane potentials were measured to determine the equilibrium concentration of Cl(-) in the tissue which could be due to diffusion. This concentration was readily exceeded in both the relatively nonvacuolate tips (0 to 1 mm) and the vacuolate, mature upper sectons (1 to 11 mm) of the roots. The activity coefficient of both cytoplasmic and vacuolar Cl(-), measured with Cl(-) sensitive microelectrodes, was approximately the same as that of a pure KCl solution of the same concentration. It is concluded that the "second mechanism" of ion uptake involves a large increase in the rate of active transport at the plasmalemma as the external concentration is increased above 1 mm.