The intervening sequence (IVS) of the Tetrahymena thermophila ribosomal RNA precursor undergoes accurate self-splicing in vitro. The work presented here examines the requirement for Tetrahymena rRNA sequences in the 5' exon for the accuracy and efficiency of splicing. Three plasmids were constructed with nine, four and two nucleotides of the natural 5' exon sequence, followed by the IVS and 26 nucleotides of the Tetrahymena 3' exon. RNA was transcribed from these plasmids in vitro and tested for self-splicing activity. The efficiency of splicing, as measured by the production of ligated exons, is reduced as the natural 5' exon sequence is replaced with plasmid sequences. Accurate splicing persists even when only four nucleotides of the natural 5' exon sequence remain. When only two nucleotides of the natural exon remain, no ligated exons are observed. As the efficiency of the normal reaction diminishes, novel RNA species are produced in increasing amounts. The novel RNA species were examined and found to be products of aberrant reactions of the precursor RNA. Two of these aberrant reactions involve auto-addition of GTP to sites six nucleotides and 52 nucleotides downstream from the 3' splice site. The former site occurs just after the sequence GGU, and may indicate the existence of a GGU-binding site within the IVS RNA. The latter site follows the sequence CUCU, which is identical with the four nucleotides preceding the 5' splice site. This observation led to a model where where the CUCU sequence in the 3' exon acts as a cryptic 5' splice site. The model predicted the existence of a circular RNA containing the first 52 nucleotides of the 3' exon. A small circular RNA was isolated and partially sequenced and found to support the model. So, a cryptic 5' splice site can function even if it is located downstream from the 3' splice site. Precursor RNA labeled at its 5' end, presumably by a GTP exchange reaction mediated by the IVS, is also described.