Potassium is a major osmolyte used by plant cells. The accumulation rates of K+ in cells may limit the rate of expansion. In the present study, we investigated the involvement of ion channels in K+ uptake using patch clamp technique. Ion currents were quantified in protoplasts of the elongation and emerged blade zone of the developing leaf 3 of barley (Hordeum vulgare L.). A time-dependent inward-rectifying K+-selective current was observed almost exclusively in elongation zone protoplasts. The current showed characteristics typical of Shaker-type channels. Instantaneous inward current was highest in the epidermis of the emerged blade and selective for Na+ over K+. Selectivity disappeared, and currents decreased or remained the same, depending on tissue, in response to salt treatment. Net accumulation rates of K+ in cells calculated from patch clamp current-voltage curves exceeded rates calculated from membrane potential and K+ concentrations of cells measured in planta by factor 2.5-2.7 at physiological apoplastic K+ concentrations (10-100 mm). It is concluded that under these conditions, K+ accumulation in growing barley leaf cells is not limited by transport properties of cells. Under saline conditions, down-regulation of voltage-independent channels may reduce the capacity for growth-related K+ accumulation.