In the present study the ribosomal RNA of the leech Hirudo medicinalis has been characterized at the aim of identifying possible analogies with other invertebrates. Upon electrophoresis on denaturating gels, ribosomal RNA fraction of H. medicinalis exhibited a remarkable thermal instability by dissociating into two hydrogen-bonded components when heated at 60 degrees C, at variance with the behaviour of the rat rRNA, which does not show this process. This result suggests a functional role in leech ribosome organisation that requires deeper structural studies.