The role of nitrate in osmotic control was studied with Italian ryegrass grown in a nutrient solution in a climate room. Quantum-flux density, osmotic potential of the nutrient solution and availability of nitrate and chloride were varied independently. Plants at high quantum flux density (650 μmol m−2 s−1) had a lower osmotic potential, a higher carbohydrate concentration and a lower nitrate concentration than plants at low quantum flux density (310μmol m−2 s−1), the decrease in nitrate concentration was osmotically equivalent to the increase in carbohydrate concentration. When nitrate in the nutrient solution was partly replaced by chloride, the chloride taken up substituted an equivalent part of the nitrate in the plant. It is concluded that nitrate plays a role in osmoregulation of the plant and compensates for a shortage of other solutes.