Labelled RNA was extracted from growing and stationary cultures of the ciliate Tetrahymena and was separated chromatographically into poly(A)- and no poly(A)-containing fractions. A new method was used to derive from the data (cpm/A260, and tD, doubling time of RNA) absolute values of three growth terms which fully describe the population kinetics of RNA molecules: the rates of transcription, decay, and net growth. At all times the messenger RNA (mRNA) content of Tetrahymena was the result of a self-regulating equilibrium between synthesis and decay. The rates of transcription and of degradation of mRNA and ribosomal RNA (rRNA) were found to be controlled independently, but decay was dominant in establishing the growth-specific quantities per cell. In the stationary phase about 94% of all poly(A)-RNA molecules and about 50% of all mRNA molecules were kinetically silent. The remaining portions were transcribed with high rates, but also degraded immediately. During the culture growth cycle the rate of rRNA net growth responded positively to the cellular rRNA content suggesting an autocatalytic effect of rRNA on the rate of its accumulation.