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In order to investigate the chemical composition of the nuclear pore complexes isolated nuclei from mature Xenopus laevis oocytes were manually fractioned into nucleoplasmic aggregates and the nuclear envelopes. The whole isolation procedure takes no more than 60-90 sec, and the pore complexes of the isolated envelopes are well preserved as demonstrated by electron microscopy. Minor nucleoplasmic and cytoplasmic contaminations associated with the isolated nuclear envelopes were determined with electron microscopic morphometry and were found to be quantitatively negligible as far as their mass and nucleic acid content is concerned. The RNA content of the fractions was determined by direct phosphorus analysis after differential alkaline hydrolysis. Approximately 9% of the total nuclear RNA of the mature Xenopus egg was found to be attached to the nuclear envelope. The nonmembranous elements of one pore complex contain 0.41×10 g RNA. This value agrees well with the content estimated from morphometric data. The RNA package density in the pore complexes (270×10 g/μ) is compared with the nucleolar, nucleoplasmic and cytoplasmic RNA concentration and is discussed in context with the importance of the pore complexes for the nucleo-cytoplasmic transport of RNA-containing macromolecules. Additionally, the results of the chemical analyses as well as of theH-actinomycin D autoradiography and of the nucleoprotein staining method of Bernhard (1969) speak against the occurence of considerable amounts of DNA in the nuclear pore complex structures. [ABSTRACT FROM AUTHOR]