The absolute and relative amounts of soluble and ribosomal RNA in rabbit reticulocytes have been determined. The proportion of total reticulocyte RNA which is membrane bound, including transfer RNA and ribosomal RNA, is approx. 22 %. In blood with a high reticulocytosis about 75 % of total reticulocyte RNA is ribosomal RNA. The total number of ribosomes in an average reticulocyte is estimated to be 3.2·10 4. About 18 % of reticulocyte ribosomes are bound to the cell membrane. The number of transfer RNA molecules is estimated to be 9.1·10 6. The relative proportions of ribosomal and transfer RNA are related to the age of the reticulocyte. As the reticulocyte matures the rate of disappearance of ribosomal RNA exceeds that of transfer RNA, suggesting that degradation of soluble RNA and ribosome fractions are independently regulated.