The action of alkaloid cytisine on protein biosynthesis in eukaryotic cells was studied. It was shown that the alkaloid had no effect on mRNA translation. Cytisine inhibited the release of mRNP particles from rat liver and wheat embryos nuclei. Sedimentation properties and distribution in CsCl gradient of the material extracted from alkaloid-treated nuclei did not differ from control one and were similar to informosomes from animal and plant cells described earlier. The main part of mRNA with sedimentation coefficients 14-18S, capable for translation in the cytoplasm is retained in alkaloid-treated nuclei.