The conformational transitions of viroid-like RNAs associated with cadang-cadang disease, velvet tobacco mottle virus, and solanum nodiflorum mottle virus were studied by melting analysis and fast temperature jump technique in 1 mM sodium-cacodylate, 10 mM NaCl, 0.1 mM EDTA, pH 6.8. The 4 circular RNAs of cadang-cadang show a highly cooperative transition between 45 and 49 degrees C, respectively, and a second transition of less hypochromicity at about 10 degrees C higher temperatures. The data are interpreted quantitatively on the basis of the sequences and secondary structure models. A very similar scheme for the structure and structural transitions as derived earlier for other viroids applies to the cadang-cadang RNAs. In the main transition the total native secondary structure is disrupted and a stable hairpin consisting of 9 base pairs is newly formed which dissociates in the second transition. The thermal denaturation of the circular RNAs from the viruses mentioned above is clearly distinct from viroid RNA in respect to stability and cooperativity. The results on cadang-cadang RNA are discussed in the light of recent hypotheses about the interference of viroids with the splicing process of the host cell.