1. The kinetics of RNA synthesis in nucleate and enucleated cells of Acetabularia mediterranea have been compared. 2. Nucleate cells and cells 24 hours after enucleation show no differences in their kinetics nor in the sedimentation behavior of the newly-synthesized RNA isolated after various short intervals (≦120 min) of incubation in the presence of tritiated ribonucleosides which can be attributed to enucleation. This fact is explained by predominant extranuclear RNA synthesis. 3. The kinetics of the synthesis of the 23 s and 16 s ribosomal RNA peaks suggest that each of these peaks may be composed of two components. The possible origin of these components is discussed. © 1969 Springer-Verlag.