A recent X-ray analytical technique for the measurement of inorganic solutes in dried vacuolar sap has been improved and tested and was used to measure the content of barley leaf epidermal vacuoles. This method was compared with a technique which measured inorganic solutes in vacuoles of frozen leaf sections. The methods gave comparable results. They were used in conjunction with a range of other microanalytical methods to investigate the role of potassium as an osmoticum in individual leaf epidermal cells of barley grown in either low (0.2 mM) or control (4.0 mM) levels of potassium. In the low potassium plants, both turgor and osmotic pressure were at first relatively low, but eventually rose to levels similar to the control plants. In both treatments, vacuolar potassium and its counterion accounted for all the osmotic pressure in young leaves. However, during leaf development, the potassium was replaced to . varying extents by calcium. During this time gradients of ion concentrations developed between adjacent cells, depending on their proximity to vascular tissue. In older leaves, the cellular concentrations of calcium and potassium were negatively correlated. The nature of this relationship was affected by potassium nutrition. In control plants, the ratio of calcium to potassium concentration was 2: 3 (0.67). This value is consistent with the maintenance of osmotic pressure which was observed. In the low potassium plants, the Ca: K ratio was 0.76, which explained the increase in osmotic pressure over time in these plants. It is proposed that the epidermis behaves as a storage tissue for potassium which is retranslocated to younger tissue as the leaf ages. The rate of export depends on the potassium nutrition of the plant. These events are also consistent with the hypothesis that turgor and osmotic pressure are regulated to compensate for the arrival of soluble calcium in the leaf.