Net photosynthetic rate, leaf area, tillering, shoot weight, and leaf area ratio for 35 F1 rice (Oryza sativa L.) hybrids during the vegetative stage were compared with corresponding values for parental varieties. Shoot weight and leaf area showed heterosis. However, hybrid vigour was not reflected in net photosynthetic rate. Leaf area was closely correlated with heteroses in shoot dry weight and tillering. Thus F1 rice hybrids grow vigorously because of their high leaf area development, which is caused by high tillering.