The regulation of K+ influx into roots of Westerwolds ryegrass (Lolium multiflorum Lam.) and white clover (Trifolium repens L.) seedlings has been studied. In low salt plants the absorption isotherms had the same Michaelis constants (Km) but ryegrass had a greater maximum rate of influx (Vmax) than white clover. When the potassium contents of the roots were increased the difference in Vmax diminished but the Kms increased, the value for white clover increasing much more rapidly than the value for ryegrass. The data gave very similar Hill plots for both species which suggests that the molecular mechanisms responsible for the regulation of potassium absorption have strong similarities. The practical implications of the findings are considered and it is suggested that under conditions where competition for potassium is important the specific rate of potassium absorption for ryegrass could be two to five times faster than that for white clover.