1. Experiments have been performed to examine methods to determine hydraulic conductivity and diffusional permeability in disks of red beet (Beta ivulgaris) and Jerusalem artichoke (Helianthus tuberosus). 2. The half-time for the weight change caused by transferring disks between pure water and a sucrose solution, or vice-versa, has been shown to be linearly dependent on the square of the disk thickness. 3. The efflux kinetics of sucrose from these disks are also characterized by a half-time linearly dependent on the square of the disk thickness. 4. It is concluded that extracellular solute diffusion, and not hydraulic con ductivity, is the principal rate-controlling factor in experiments of this type. 5. Efflux studies with tritiated water also show a characteristic half-time which is linearly dependent on the square of the disk thickness. The cell membrane does not constitute a sufficient barrier to diffusion for its diffusional permeability to be determined.