NUMEROUS attempts have been made to deduce a secondary structure of 5S RNA1–4. Differing approaches have yielded various structures, but each turns out to be inconsistent with either physical, chemical, biochemical, or comparative evidence. Clearly the situation is not analogous to that which obtained for transfer RNA (tRNA). There is, however, at least one fundamental difference between the two molecules. The functional 5S RNA molecule is at all times an integral part of the 50S ribosomal subunit, whereas tRNA is a transient inhabitant of the ribosome milieu. Thus, it is possible that functional conformations of the 5S RNA molecule need only exist in the context of the ribosome, and consequently, may only exist therein. Thus, the interpretation of experimental investigations on the isolated 5S RNA molecule may have inherent difficulties.