The time course of the uptake of labelled calcium by roots of Hordeum vulgare L., Trifolium subterraneum L., and Phaseolus aureus Roxb. was found to have two distinct phases: an initial rapid adsorption phase of about 1 h duration (Phase I) and a slower accumulation which proceeded at a constant rate for at least 12 h (Phase II). Ions taken up by the mechanism of Phase I, but not Phase II, could be removed by rinsing the roots in cold solutions of unlabelled CaCl2 or KCl.The concentration dependence of calcium uptake by barley roots over the range 0.001 to 10.0 mN was analysed with the aid of Hofstee plots. Phase I consisted of two Donnan adsorption processes. The half-saturation concentrations were 0.065 mN and 0.002 mN and the respective capacities were 1.56 and 0.36 μeq g(-1) (fresh weight). Phase II displayed Michaelis-Menten kinetics for a single mechanism with a Km=0.077 mN and V max=0.15 μeq g(-1) (fresh weight) h(-1).