Uptake of aluminium by excised roots of cabbage, lettuce, kikuyu grass and by a weakly acidic cation exchange resin was studied. The role of metabolism was also investigated, using a range of temperatures and a metabolic inhibitor. The time course of uptake had two phases: an initial rapid phase of about 60 min duration for all species and a slower accumulation phase that was pronounced for cabbage and lettuce and almost absent for kikuyu grass. During the initial phase, calcium was exchanged by aluminium. A small fraction of the aluminium taken up by excised roots could not be desorbed by dilute buffer; this was thought to result from irreversible binding of aluminium directly to exchange sites. Evidence is also presented for a third component for aluminium uptake involving diffusion through the plasmalemma. The non-metabolic nature of the entire uptake process was confirmed by the negligible effect of temperature (1-30°C) and enhancement by 2,4-dinitrophenol on the rate of uptake. Aluminium uptake at pH 4.2 exceeded that at pH 4.0, probably due to the lowering of net charge density of aluminium ions as pH is increased.