The relationship between fruit load and root activity, vegetative growth and sugar accumulation was investigated in large-potted grapevines, 'Aki-queen' (Vitis viniferaxVitis labrusca) with heavy (two clusters per fruit bearing shoot), medium (one cluster per shoot) and light (non-fruiting) fruit loads grown in a non-heated plastic vinyl house. Root activity was evaluated by the stable isotope 15N (ammonium sulfate-15N) uptake rate and respiratory rate of fine roots using the liquid phase oxygen electrode method. New roots and lateral shoot growth, sugar content in berries and the dry weight of each organ were also measured. Respiratory rate of fine roots and 15N-labelled nitrogen uptake in light fruit load (LFL) grapevines were higher than those of heavy fruit load (HFL) and medium fruit load (MFL). HFL reduced lateral shoot growth and the production of new fine roots during this experiment. In addition, very small new fine roots were found in HFL and MFL grapevines after the veraison stage at the beginning of July although there were new fine roots on LFL grapevines throughout the growth period. The weights of both clusters and berrie, and the sugar content of berries were significantly decreased by heavy fruit loads at harvest stage in late-August. However, total sugar accumulation relative to berries per tree was highest in HFL grapevines due to there being more clusters per vine. These results indicate that root activity and vegetative growth were strongly affected by the variation in fruit load, and that a heavy fruit load forced vines to accumulate sugar in the berries prior to partitioning photoassimilates in leaves, shoots and roots.