The contribution of the starch and soluble carbohydrate pools to the diurnal variations of leaf mass per unit area (LMA) has been investigated in tomato leaves. A glasshouse experiment was carried out with plants pruned to two or five fruits per truss. Leaflets were sampled at sunrise, noon and sunset at different positions within the leaf (basal or terminal), and on different sympods along the stem. Carbohydrate contents and LMA were significantly higher in the terminal than in the basal leaflets, except at sunrise. During the day, differences in starch accumulation between terminal and basal leaflets increased with leaf height on the plant. Among sympods, the soluble carbohydrate content of the terminal leaflets did not vary significantly, whereas at 13.00 h the LMA was minimum in the middle of the plant and maximum at the top, and the leaf starch content significantly increased half-way up the plant. The plant fruit load had only small and non-significant effects on the LMA and carbohydrate contents. The response of LMA and carbohydrate contents to changing source activity was observed under controlled climatic conditions. The starch pool of fully expanded leaves was rapidly filled and emptied under increasing and decreasing source activity. In young expanding leaves, this pool was hardly filled during daylight. On average the soluble carbohydrates did not contribute significantly to the diurnal variations in LMA, whereas fluctuations in starch explained c. 70% and 44% of these variations in the upper and lower leaves, respectively. The results are discussed with respect to the modelling of LMA at the level of individual tomato leaves or sympods.