The effect of emetine, an inhibitor of cellular protein synthesis, on young and old cell lines of Paramecium aurelia was studied. Emetine treatment resulted in the accumulation of ribosomal aggregates in the form of helices or rosettes. In treated young cells, large patches of endoplasmic reticulum with clusters of ribosomes were frequently observed. These structures were smaller in treated aging cells. Owing to the association of these ribosomal clusters with endoplasmic structures, they were called "ribosomal complexes". In young cells the polysome helices were more numerous, compared to those of old cells. The difference in the number of polysomes between the young and old cells appeared to be related to the number of ribosomes. The results suggested that in aging cells there were fewer ribosomes and less RNA. In very old cells the ribosomal particles appeared to be altered morphologically.