In short days of 8 h light, the glucose, fructose, sucrose and starch contents in the primary leaves and petioles of spinach plants (Spinacia oleracea L. cv. Nobel) increase during the light and decrease during the dark. The transfer to continuous light results in a 5-8 fold increase in the hexose content, which begins 1-3 h after the end of the SD light period and reaches its maximum value after 16 h of total light. The sucrose content of petioles follows the same time-dependent changes, but to a lesser extent. In contrast, the malate and starch contents increase almost linearly with the day length. The sudden increase in hexose content can only be generated if light is given during a particular phase of the carbohydrate metabolism. The results show that part of the sugar metabolism in spinach leaves and petioles is under photoperiodic control and is endogenously organized in time.