The uptake, accumulation, and transport of potassium by excised roots of Ricinus communis were investigated. It was found that: 1. The transport of potassium to the vessels was closely dependent on the supply of the ion in the medium. 2. The flux of potassium to the vessels was only a fraction of the total potassium taken up and 99 per cent, of that absorbed was accumulated in the root tissues. Once retained in this way potassium appeared to become unavailable for transport to the vessels. 3. There was a fixed relationship between the potassium concentrations of the medium and the exudate when potassium was supplied as potassium nitrate in the medium at a range of concentrations. This relationship was altered by the presence of other ions in the medium.