Chemical modification of uridine residues in yeast RNA was carried out at two different concentrations of hydroxylamine, 10 M and 5 M, pH 10 and 37℃, varying the reaction time. The degree of modification depended on the concentration of hydroxylamine : the range was ca. 4∼5% per hour at 10 M hydroxylamine, and 2% per hour at 5 M hydroxylamine. Modification rates decreased with the reaction time, probably due to the limiting effect exerted by the secondary structure of RNA. The modification reaction was highly specific to uridine residues under the above reaction condition, so that the modification reaction on cytidine residues was almost negligible. It is found that less than 1% of uridine residues was modified at pH 10 and 10 M of hydrnxylamine within 20 min. This reaction condition can be utilized for RNA sequencing in-chemical methods.