Hybrid rice parental lines V20B(an easily-sprouting line) and Jin 23B(a hardly-sprouting line) were used as the experimental materials to study the physiological basis of preharvest sprouting of rice grains.The results showed that V20B was easier to sprout than Jin 23B,and the easily-sprouting V20B had a greater ratio of the content in grains of GA3 to ABA and a lower ABA content in grains before preharvest sprouting.In addition,in the grain development V20B had a higher α-amylase activity and a higher water content in grains than Jin 23B.