The 5' termini of 18S and 28S rRNA and their 32S, 41S, and 45S nRNA precursors were analyzed by fractionating alkaline hydrolysates of the RNAs on columns of diethylaminoethyl-Sephadex A-25 at neutral pH followed by electrophoresis on Whatman 3MM paper at pH 3.5. The results indicate that each RNA begins with a mononucleoside diphosphate of heterogeneous composition. The principal nucleoside diphosphate was pCp in 28S, 32S, 41S, and 45S RNA and pUp in 18S RNA. The 18S rRNA also contained the most homogeneous composition with approximately 70% of the molecules beginning with pUp; the 41S and 45S nRNA termini were the most heterogeneous, each containing pCp in only one-third of the molecules. Each terminal composition was reproducible and characteristic for its RNA. The composition of 28S rRNA was very similar to that of 32S nRNA, but distinctly different from 5.8S or 18S rRNA or 45S nRNA. The isomeric forms of 45S nRNA contained no appreciable amounts of a terminal 5'-triphosphate, although they had similar nucleoside diphosphate compositions. These results are discussed in relation to the transcription of ribosomal genes and the arrangement of the 5.8S rRNA sequence within these genes.