With the view to elucidate the role of myo-inositol in the ripening process of rice grains, its distribution, formation and conversion were studied.myo-Inositol in the ripening rice grains was fractionated into free-, phosphate ester- and phosphoinositide-forms. At the early stage of ripening, a considerable part of myo-inositol was found in free state, and at the end of ripening stage the most part was found in phosphate ester-state, phytic acid. The content of phosphoinositide in the grains was low during the ripening period.The occurrence of biosynthesis of myo-inositol in the ripening rice grains was confirmed by the observation of incorporation of ¹⁴C into myo-inositol from ¹⁴C-sugars and it was found, from the feeding experiment of myo-inositol- that myo-inositol does not undergo reactions further than phosphorylation.The feeding experiment of glucose-l-³²P showed that the distribution pattern of ³²P in different fractions of grain material was the same as that of ³²P-phosphate, indicating that phytic acid is one of the final products of phosphorus metabolism in the ripening rice grains.These results led to the assumption that myo-inositol might act as an acceptor of phosphorus to remove inorganic phosphorus in favor of starch synthesis by phosphorylase.