Résumé : Nucleoli of HeLa cells growing in tissue culture were selectively irradiated with an u.v. microbeam and the subsequent incorporation of [3H]cytidine into the ribonucleic acid of the nucleolus, extra-nucleolar parts of the nucleus, and the cytoplasm, was studied by autoradiography. Comparison with cells containing normal nucleoli revealed that approximately 2 3 of the cytoplasmic incorporation is directly dependent on the nucleolus, but that less than 1 3 of the incorporation into extra-nucleolar parts of the nucleus is dependent on the nucleolus. These results when combined with quantitative kinetic studies of cytidine incorporation strongly support the hypothesis that a major part of the ribonucleic acid of the cytoplasm is synthesized in the nucleolus. © 1961.