Ribosomal ribonucleic acid (rRNA) from Escherichia coli has been oxidised with sodium periodate and then reacted with [ carbonyl- 14C]isonicotinic acid hydrazide, to form a derivative in which the 3′-hydroxy terminal ribosyl group of the rRNA is modified and combined with the isonicotinic acid hydrazide (INH). Various fractions of such rRNA derivatives have been degraded by specific ribonucleases, when nucleoside- or oligonucleotide-isonicotinyl hydrazones were produced. These were subjected to high-voltage electrophoresis and labelled materials were identified by comparison with known standards. The results indicated that E. coli rRNA is terminated mainly in two sequences which make up about 80 % of the total. Bacterial cultures, grown to the stationary phase in glucose-salts media, possessed total rRNA with 43 ±2 % terminal adenylic acid (A) and 38±2 % terminal uridylic acid (U). The existence of 3′-hydroxy terminal A and U in rRNA in the above proportions was then confirmed by careful alkaline hydrolysis of specifically labelled rRNA, when the chief nucleosides produced in digests were adenosine and uridine. The combination of these findings with the results of experiments where rRNA-[ 14C]isonicotinyl hydrazones were degraded with T 1 or pancreatic ribonucleases suggested that the terminal sequence of “A-terminal” rRNA's was principally -pGpCpA and of “U-terminal” rRNA's was -pPypU. From studies on isolated 16-S and 23-S rRNA fractions, it seems that 16-S rRNA is much richer in “A-terminal” chains than in “U-terminal” chains. The converse is true of 23-S rRNA, although the distribution of “A-terminal” and “U-terminal” species in these two rRNA's suggests that entities with both termini occur naturally in each fraction. It was also found that rRNA of high purity (99.95 % with respect to protein) gave a final reactivity to periodate and [ 14C]INH corresponding to a polynucleotide of about 1500 nucleotide residues. This was true of either total rRNA, 16-S or 23-S specimens. 23-S rRNA was unstable after periodate oxidation, and was converted to material almost indistinguishable from 16-S rRNA. rRNA of only 99.0–99.5 % purity had a reactivity to periodate and [ 14C]INH which corresponded to a polynucleotide of 2150 residues. From such rRNA, 16-S material, purified by sucrose density-gradient sedimentation, had a reactivity corresponding to an rRNA of 1550 nucleotide residues, and 23-S material, a reactivity corresponding to an rRNA of 2800–3500 residues. However, this 23-S fraction, like that of more purified rRNA, was also unstable to periodate. It is considered that earlier results, which indicated that 23-S rRNA had only one periodate-oxidisable group per 3000 nucleotide residues, were due to interference of trace contaminants which tended to sediment preferentially with 23-S rRNA in purification procedures. No evidence can be found for a covalent binding of 16-S subunits in 23-S rRNA which involve periodate-oxidisable 3′-hydroxy terminal groups. Alternative theories are proposed to bring together the implications of physicochemical and chemical estimations of the polynucleotide chain length of E. coli rRNA's.