Mouse globin messenger RNA has been degraded by ribonuclease T, to give poly(adeny1ic acid) fragments of two size classes which run electrophoretically with standard RNA molecules approximately 75 and 50 nucleotides long. These contain no internal ribonucleotides other than adenosine and occur at the 3'-end of the mRNA molecule. The probable sequence to the 5'-side of the poly(A) tract is -G-C. Attempts to separate the DL and B mRNA activities by formamide gradient elution from poly(U)-Sepharose were unsuccessful.The evidence is more compatible with a post-transcriptional end-addition mechanism for t,he synthesis of the poly(adeny1ic acid) region of messenger RNA than a transcriptional mechanism.