Tritiated ribosomal RNA (rRNA) was prepared from the roots of Vicia faba after incubation in 3H uridine. Separation of the nucleic acids MAK chromatography yielded fractions of specific activity of 4-5 x 105 dpm/μg. 4 + 5S, 18S and 25S RNA fractions were used for cytological hybridization on squash preparations of Vicia faba root tip meristems. Autoradiographs of the 18S and 25S RNA preparations exhibited a clear labelling in the secondary constriction of the satellite (SAT) chromosomes after exposition times of 28 weeks.