Abstract A high molecular weight, rapidly-labelled ribosomal RNA precursor in the pearoot tip and in artichoke-tuber tissue is described. It has a molecular weight, determined by polyacrylamide-gel electrophoresis, of about 2.3 × 10 6 daltons. The pea precursor RNA consists of two components which can just be distinguished by gel electrophoresis, whereas that from the artichoke appears homogeneous. 2.3 × 10 6 daltons RNA is labelled within 15 minutes of incubation in [ 32 P]orthophosphate, and is followed by a second precursor of molecular weight l.4 × 10 6 daltons. Also appearing at this time is a third component of 0.9 × 10 6 daltons. The base composition of the 2.3 × 10 6 and 1.4 × 10 6 daltons RNA's is similar to that of ribosomal RNA, in contrast to heterogeneous RNA. In older regions of the pea root, in which the rate of ribosome synthesis is low, the synthesis of precursor molecules is decreased. In excised pea-root tips, cultured in vitro , in which there is no net protein or RNA synthesis, label is incorporated into the precursor. Assembly of ribosomes, however, is prevented by inhibition of its processing, with accumulation of the 2.3 × 10 6 , 1.4 × 10 6 and 0.9 × 10 6 daltons components. The method of synthesis of ribosomal RNA in plants is discussed in relation to that in animals.