A simple model has been devised which predicts the concentration, Cxs, of salt (e.g. KCl) in the exudate from isolated roots as a function of the salt concentration, C0s, in the medium. The chief assumption, made in deriving the relationship between Cxs and C05, is that the exudation of water, Jω, from the roots consists of two components (one being osmotic, Øω, in origin and the other, Ø0ω, flowing in the absence of an osmotic gradient). The exudation of salt, Js, calculated as JωCxs, was found to be dependent on C0s. Our investigations on maize roots were concerned with estimations of Lp and Øvω and determinations of Cxs as a function of C0s. Satisfactory agreement between prediction and experiment was found in these preliminary studies. It is considered that water movement in isolated roots can be explained by a simple osmotic model with the additional possibility that a relatively small non-osmotic water flow occurs.