Five pre-23 S rRNA introns have been characterized for hyperthermophilic archaea. The small ones constitute stable core structures while the three larger ones contain, in addition, open reading frames, two of which encode homing-type endonucleases. The higher order structures of in vitro transcripts of the introns and their exon-intron junctions were examined using chemical and ribonuclease probes specific for unpaired nucleotides, double helical regions and helix-loop junctions. The experimental data support both the formation of a "bulge-helix-bulge" structural motif at the exon-intron junctions, which is recognized by a cleavage enzyme, and the presence of an adjacent core structure consisting of a long, stable, stem-loop structure. Moreover, the data were used to derive secondary structural models for the three large introns which form stable, circular, RNA species, in vivo. Each exhibits extensive secondary structure and additional, as yet undefined, higher order structure, compatible with their high level of stability in vivo; limited structural similarities were detected between the two introns encoding the homing-type endonucleases. The data are compatible with the hypothesis that RNA introns can be transmitted between archaeal hyperthermophile cells.