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On the basis of two types of calcium transport system detected in the barley root plasmamembrane,the mechanisms of the calcium transport have been further studied.Ionophore CCCP has beenfound to inhibit Mg~(2+) -dependent calcium transport by 20%.In contrast,Mg~(2+) -independent calcium trans-port is insensitive to CCCP.The Mg~(2+) -dependent calcium transport following the collapse of H~+ gradientacross the plasma membrane could be driven by the H~+ gradient either set up by ATP or imposed artificially.Any relation between Mg~(2+) -independent calcium transport and H~+ gradient has not been observed.Theseresults indicate that Mg~(2+) -dependent calcium transport is accompanied by the decrease of H~+ gradient,andMg~(2+) -independent calcium transport has nothing to do with the H~+ gradient.It is therefore suggested thatthe calcium transport across the barley root plasma membrane is driven by ATPase that is independent ofMg~(2+),and H~+/Ca~(2+) antiporter that is dependent on Mg~(2+).