Drought of all environmental factors limits globally crop production most severely. Sucrose is the major transport carbohydrate in most plants. Under drought, the grain growth is limited due to the availability of carbohydrates. Even short periods of drought usually cause either abortion of flowers and grains or decrease grain filling. Our aim was to study the regulatory aspects of sink and source processes by linking the drought induced changes in carbon dynamics to growth, source versus sink limitation and yield determination. The experiment was conducted in 2006 at the MTT Agrifood Research, Finland, in a greenhouse. Wheat (cv. Amaretto) was sown on April with 350 grains m^-2. Plots were fertilized with 120 kg N ha^-1 (N-P-K:20-4.8-8). The day/night temperatures were 20-30°C/10-20°C. Photoperiod was approximately 14 h. Control plots were watered frequently throughout the growing season, whereas drought was imposed at anthesis by withhelding watering. The experiment was conducted in a randomised complete block design with three replications. Photosynthesis, stomatal conductance, water potential and osmotic potential was measured from the flag leaves once a week. Samples for biomass accumulation were collected every second week. Head samples for starch, sucrose and invertase localisation, and determination of endosperm cell number were collected every second day after pollination until maturity. Drought resulted in lower water potential and photosynthesis and increased osmotic potential. At the beginning of senescence the photosynthesis of control plants also began to decrease. Drought decreased the biomass accumulation as well as number of fertile florets and grains. The number of active nuclei declined slower in well-watered than in drought treated plants. There was no obvius difference in starch accumulation of filling grains under drought or control. However, at early stage after pollination some grains withered under drought. Drought reduce yield by both decreasing availability of assimilates and storage potential.