(1) The turnover of ribosomal RNA (r-RNA) in mouse peritoneal macrophages was studied in a cell culture system, where macrophages did not multiply but remained alive in vitroin vitro for at least a week. Macrophages were incubated with 3H-uridine for 24 hr to allow uniform labeling of RNA's. They were then transferred to fresh medium containing non-radioactive uridine and chased for 4 days. The specific activity of RNA decreased logarithmically with time, the half-life being about 48 (36–70) hr. The cellular RNA content remained constant and the synthetic activity of RNA did not change greatly during this time. This result shows that there is a turnover of the RNA. (2) The RNA species undergoing metabolic turnover were found to be mainly ribosomal RNA's by sucrose density gradient centrifugation.