A new method for the labeling of terminal cis-diol end groups in ribopolynucleotides is described; it involves the reaction of an aqueous solution of RNA with sodium metaperiodate and subsequent reduction of the resulting terminal dialdehyde with 3H-sodium borohydride. The reaction conditions are mild, and conversion to end group-labeled RNA is nearly quantitative, as judged by the incorporation of radioactivity into pApApA and yeast transfer RNA (tRNA). With pApApA and tRNA as substrates, the specificity of reaction for 2′,3′-diol end groups has been confirmed. This method has been applied to the sequence analysis of an octanucleotide isolated from yeast phenylalanine tRNA. A general scheme for 3′-terminal sequence analysis of ribopolynucleotides by application of this technique is proposed.