Stable secondary structures in 3 0 UTRs have been shown to play a role in mRNA sorting and localization (20) . Some of them have been also reported to influence mRNA stability (21) . The existence of a stable structural motif in the 3 0 UTR of the rat Pol b mRNA was predicted previously (12) and it was speculated that it may have a regulatory role. In the present work, secondary structure analysis by lead ion hydrolysis and enzymatic digestion revealed the existence of several motifs in the analyzed sequence, namely, a region of strong lead ion binding (M1), a hairpin-forming and highly evolutionarily conserved region (designated as M2) and the region containing polyadenylation sequence (M3). Only M2 is evolutionary conserved, the rest of the untranslated region has high interspecies sequence variation, which suggests lesser functional importance. The sequence in the conserved region exhibits 86.8% identity to the homologous human sequence (100% in the upper stem region), which may indicate a similar role of this element in human cells.