Plants can use ammonium (NH1 4 ) as the sole nitrogen source, but at high NH1 4 concentrations in the root medium, particularly in combination with a low availability of K1, plants suffer from NH1 4 toxicity. To understand the role of K1 transporters and non-selective cation channels in K1/NH1 4 interactions better, growth, NH1 4 and K1 accumulation and the specific fluxes of NH1 4 , K1, and H1 were examined in roots of barley (Hordeum vulgare L.)and Arabidopsis seedlings. Net fluxes of K1 and NH1 4 were negatively correlated, as were their tissue concentrations, suggesting that there is direct competition during uptake. pharmacological treatments with the K1 transport inhibitors tetraethyl ammonium (TEA1)and gadolinium (Gd31)reduced NH1 4 influx, and the addition of TEA1 alleviated the NH1 4 -induced depression of root growth in germinating Arabidopsis plants. Screening of a barley root cDNA library in a yeast mutant lacking all NH1 4 and K1 uptake proteins through the deletion of MEP1-3 and TRK1 and TRK2 resulted in the cloning of the barley K1 transporter HvHKT2;1. Further analysis in yeast suggested that HvHKT2;1, AtAKT1, and AtHAK5 transported NH1 4 , and that K1 supplied at increasing concentrations competed with this NH1 4 transport. On the other hand, uptake of K1 by AtHAK5, and to a lesser extent via HvHKT2;1 and AtAKT1, was inhibited by increasing concentrations of NH1 4 . Together, the results of this study show that plant K1 transporters and channels are able to transport NH1 4 . Unregulated NH1 4 uptake via these transporters may contribute to NH1 4 toxicity at low K1 levels, and may explain the alleviation of NH1 4 toxicity by K1.