α-Amylase isoforms I-1 and II-4 were found in rice grains during ripening, α-amylase II-4 being the most predominant isoform. To determine their functions in ripening seeds, we generated a series of transgenic rice plants transformed with α-amylase I-1 and α-amylase II-4 cDNA under the control of Cauliflower mosaic virus 35S promoter. These isoforms were increased in young shoot and mature leaf tissues of the transgenic plants at both mRNA and protein levels. The starch accumulation in leaves was reduced to 42-82% of that in the wild-type. The transgenic lines A3-1 and D1-4, which overexpressed α-amylase I-1 and α-amylase II-4, respectively, were examined further. The enzyme activity was increased in both seeds, and the increase was greater in D1-4. The dry weight of A3-1 and D1-4 seeds was decreased approximately 4 and 11%, respectively. White immature grains frequently appeared in both lines, with severer abnormalities seen in D1-4. These results strongly suggest that the increase of α-amylase activities inhibits the accumulation of reserve starch and lowers the grain quality of rice.