Rice plants were water-cultured with a combination of aluminum (0, 0.2, 0.8 mmol L^<-1>) and silicon (0, 3.0 mmol L^<-1>) in a 0.5 mmol L^<-1> Ca(NO_3)_2 solution, and both elements with different chemical forms in the plant organs were determined to evaluate the effects of silicon supply on the amelioration of aluminum injury to the plants. This culture solution was renewed every day during the treatment period (4 d). Plant growth, especially root growth, decreased with the increase of aluminum (Al) concentration in the culture solution, but the silicon supply alleviated plant growth. Total Al, water-soluble polymer Al and water-insoluble polymer Al contents in the roots and leaves tended to increase more in the presence of silicon than in the absence of silicon, and water-soluble monomer Al content in the leaves decreased. These results suggest that the ameliorative effects of silicon on aluminum injury to rice may be attributed to ; (1) growth improvement by silicon itself, (2) a decrease in aluminum concentration due to silicon in the culture solution and (3) the alteration of aluminum chemical forms in solutions and plants by silicon, resulting in a decrease in aluminum toxicity.