It has been known for sometime that AATAAA (Proudfoot and Brownlee, 1976) is often required for 3' mRNA processing. More recently, McLauchlan et al. (1985) have shown the high incidence of YGTGTTYY (Y is a pyrimidine) downstream from the poly (A) addition site. Our results fully support their findings. There have been indications that additional sequences are required either for transcription termination of mRNA or for its cleavage/processing. Here I present the results of detailed analysis of mammalian DNA sequences around the 3' ends of mRNAs. The distributions of 256 quartets and some pentamers have been studied. Except for the AATAAA several additional signals emerge, namely, the homooligomers A4-5, T4-5 and C4-5 C3-4 interspersed with a single T, alternations of T and G, TTCTT and CGAGG. These are highly regular sequences which may exhibit unique conformations. The results also show a clear-cut asymmetry in the distribution of complementary oligomers on the same DNA strand. The possibility that at least some of these signals are recognized on the DNA and thus play a role in the termination process is discussed.