Ribose nucleic acid (RNA) and deoxyribose nucleic acid (DNA) have been localized in micrographs of Triturus and Drosophila tissues, with the use of ferric ion binding In Triturus liver and pancreas nuclei, RNA-containing material was distributed in the nucleoplasm, surrounding areas of compact DNA-containing chromatin. Where nucleoplasm was in contact with the nuclear membrane, discontinuities in the membrane (annuli) were apparent. RNA in the nucleoplasm was associated with a finely filamentous component. No structures of comparable morphology were evident in the RNA-containing structures of the cytoplasm. — RNA was also localized in specific bands of Drosophila salivary chromosomes. These areas were prepared for study by osmium fixation prior to smearing in acetic acid. Ferric binding demonstrated a finely filamentous or reticular structure in the RNA-containing regions. In the abscence of obvious morphological similarities between the RNA-containing components of nucleus and cytoplasm, we have concluded that these fractions are distinct, or that aggregation changes occur during nuclear — cytoplasmic exchange.