The uncoupler, carbonyl cyanide m-chlorophenyl hydrazone (CCCP) is shown to reduce the hydraulic conductivity of barley, maize, mung bean, and onion roots. In barley and maize, the reduction in exudation from excised roots is partly due to the reduction in the permeability of the root to water (I(p)), but it can be inferred that the rate of salt release to the xylem, is also inhibited. The action of CCCP on L(p) is suggested to be mainly in blocking the symplasmic pathway at the plasmodesmata.