The mutual interactions in the absorption of NH4, K, and Na studied in part in intact wheat plants and in part in excised wheat roots were investigated by means of the continuous flow technique. The phenomena found are: 1) The K absorption is strongly inhibited by NH4, while conversely K has little, if any, effect on the rate of NH4 absorption. 2) NH4, even at much increased concentrations, does not completely inhibit the NH4 absorption. 3) The decrease in the K absorption rate caused by NH4 at a constant K concentration and the NH4 uptake rate both show the same relation to the NH4 concentration. 4) In spite of the fact that it has a distinct effect on the absolute magnitude of the K absorption rate, the nature of the relation between the K uptake rate and the K concentration is not influenced by NH4. 5) Points 1 to 4 also apply to the NEU-Na and the K-Na interaction. NKU and K both act as inhibitors with respect to Na. 6) The NH4 absorption rate at a given NH4 concentration is strongly decreased by a rise in the nitrogen content of the root, whereas the ratio “rate of NH4 uptake/ decrease K absorption rate caused by NH4” undergoes no change. The rate of the K absorption in the absence of NH4 is not affected by the nitrogen content. 7) The ratio “NH4 absorption rate/decrease K uptake rate” is also always the same in different parts of the root in spite of differences in absolute magnitude of the rates of absorption of all three ions between the different parts. The same holds for the ratio “NH4 uptake rate/decrease Na absorption rate”. 8) The ratio “absorption rate inhibitor/decrease rate of uptake of the inhibited ion” in each of the three interactions is, where studied in the present experiments, also the same at different pH and temperature values. The explanation of these results is based on the carrier hypothesis. It is shown that: a) One mechanism is involved in the absorption of NH4, K, and Na individually. b) The absorption rate of the inhibiting ion and not the concentration of the inhibitor in the experimental solution determines the inhibitions found. It is therefore considered that the interactions do not appear at the first binding to the carrier of the ions to be absorbed but in a later phase of the absorption process.