Glucose, fructose and sucrose are the main soluble sugars present in spinach petioles. A rapid extraction method and a very sensitive (<50 pmoles) assay are described. The soluble sugars content increases during the light phase of the short day (8 h light: 16 h dark) and decreases in darkness. If the length of a short day's light phase is extended, the glucose and fructose level rises dramatically (580 and 900%, respectively). Sucrose and starch exhibit different stimulations.