Absorption isotherms for chloride and rubidium ions have been determined through a wide concentration range for nonvacuolate root tips, and for vacuolate subapical sections of corn root. In the range 0 to 0.5 mm, chloride absorption is hyperbolic with concentration in both tips and proximal sections. At high concentrations, 1 to 50 mm, a second multiple-hyperbolic isotherm for chloride is noted in vacuolate tissue, while the isotherm for nonvacuolate tips rises exponentially. A linear to exponentially rising isotherm is taken to signify diffusive permeation.The same distinction between tip and subapical tissue characterizes Rb absorption. Rb uptake is indifferent to the nature of the counterion at all concentrations in the tip, while the counterion exerts a predictable influence on Rb absorption in proximal tissue. The effect of a poorly absorbable anion on Rb uptake is greater in the high concentration range. Evidence is presented for the metabolic nature of ion transport into nonvacuolate root tips. Verification is offered that ion uptake is mediated by dual mechanisms, and the thesis is developed that the high-affinity (low K(s)) system mediates ion passage through the plasma membrane while the low-affinity (high K(s)) system implements transport through the tonoplast.