Xenopus oocytes injected with human U2 snRNA genes synthesize mature U2 as well as a U2 precursor with about 10 extra 3 nucleotides (human pre-U2 RNA). Formation of the pre-U2 3 end requires a downstream element located between position +16 and +37 in the U2 3 -flanking sequence. The distance between this element and the U2 coding region can be increased without affecting formation of the pre-U2 3 end. When the natural sequence surrounding the pre-U2 3 end is changed, novel 3 ends are still generated within a narrow range upstream from the element. The 3 terminal stem-loop of U2 snRNA is not required for pre-U2 3 end formation. A sequence within the 3 element (GTTTN0-3AAAPuNNAGA) is conserved among snRNA genes transcribed by RNA polymerase II. Our results suggest that the 3 ends of pre-U2 RNA and histone mRNA may be generated by related but distinct RNA processing mechanisms.