Plant roots accumulate K + from micromolar external concentrations. However, the absence of a firm determination of the trans-plasma-membrane electrochemical gradient for K + in these conditions has precluded an assessment of whether K+-accumulation requires energization in addition to the driving force provided by the inside-negative membrane electrical potential (Em). To address this question unequivocally, we measured Em, and the cytosolic and external K +-activities in root cells of Arabidopsis thaliana (L.) Heynh. cv. Columbia in conditions in which net K +-accumulation occurs at low external K + (10 ~tM). In these conditions, net K +uptake was about 0.1 pmol 9 (g FW) 1. h-l, Em varied between -153 and -129 mV and the cytosolic K+-activity, determined with K+-selective electrodes, was 83+4 raM. These values yield an outwardly-directed driving force on K + of at least 6.5 kJ 9 tool 1. Only if external potassium is raised to the region of 1 mM does E m become sufficient to drive net K+-accumulation. It is therefore concluded that at micromolar external K +-activities which prevail in most soils, K+-uptake cannot be solely energized by E m -as exemplified by a channel-mediated mechanism. The nature of the energization mechanism is discussed in relation to processes operating in fungal and algal cells.