5 S RNA interactions with transcription factor protein A (TFIIIA) in 7 S particles from Xenopus laevis oocytes (Xlo) have been characterized by the use of an in vitro RNA exchange assay. 32P-labeled Xlo 5 S RNA can rapidly be incorporated into 7 S particles by simple incubation of the RNA with intact particles. Incorporation of the labeled RNA during exchange reaches an equilibrium within 20 min at 20 degrees C. Labeled Xlo 5 S RNA already incorporated in 7 S particles can be chased out by an excess of unlabeled 5 S RNA. Nondenaturing gel electrophoresis of 7 S particle samples segregates several ribonucleoprotein particles containing TFIIIA and 5 S RNA. Time course experiments reveal incorporation of 32P-labeled 5 S RNA first in a higher molecular weight ribonucleoprotein particle before incorporation into the 7 S particle. In the exchange process, the integrity of the higher order structure of the RNA is essential for a recognition of the 5 S RNA by TFIIIA. Denatured Xlo 5 S RNA exchanges poorly in the presence of EDTA, but can exchange into the particle at a high level if sufficient divalent cations are present to allow the higher order structure of the RNA to reform. Xlo 5 S RNA fragments that have the 5' or 3' ends deleted past helix I markedly lose their ability to exchange. Heterologous eukaryotic and eubacterial 5 S RNAs can exchange into 7 S particles, although the eubacterial 5 S RNAs exchange at a low level.