Plant roots accumulate K + over a range of external concentrations. Root cells have evolved at least two parallel plasma-membrane K + transporters which operate at millimolar and micromolar external [K+]: high-affinity K + uptake is energised by symport with H + while low-affinity uptake is assumed to occur via ion channels. To determine the role of ion channels in lowaffinity K + uptake, a characterisation of the principal K+-selective ion channels in the plasma membrane of Arabidopsis thaliana (L.) Heynh. cv. Columbia roots was undertaken. Two classes of K+-selective channels were frequently observed: one inward (IRC) and one outward (ORC) rectifying with unitary conductances of 5 pS, 20 pS (IRCs) and 15 pS (ORC), measured in symmetrical 10 mM KC1. The dominant IRC (5 pS) and ORC (15 pS) were highly cation-selective (Pc1:PK < 0.025) but less selective amongst monovalent cations (PNa:PK ~ 0.17-0.3). Both the IRC and the ORC were blocked by Ba 2+, Cs + and tetra-ethyl-ammonium, whereas 4-aminopyridine and quinidine selectively inhibited the ORC. The ORC open probability was steeply voltage-dependent and ORC activation potentials were close to the potassium equilibrium potential (EK+), enabling ORCs to conduct mainly outward, but occasionally inward, K + current. By contrast, gating of the 5-pS IRC was weakly voltageependent and IRC gating was invariably restricted to membrane potentials more negative than EK+, ensuring K § transport was always inwardly directed. Studies on channel activity were conducted for a large number of root cells grown at two levels of external [K +], one where K § uptake is likely to be principally through channels (6 mM K +) and one where it must be energised (100 taM K +). Shifting growth conditions from high to low K + did not affect single-channel properties such as conductance and selectivity, nor the manifestation of the ORC and 20-pS IRC, but led to enhanced activity of the 5-pS IRC.