The cell cycle-related synthesis rate and accumulation of RNA and protein were studied in in vivo growing Ehrlich ascites tumour cells. After pulse-labelling in vivo, cell fractions from various parts of the cell cycle were obtained by means of elutriator centrifuging. The RNA synthesis rate increased three times during G1 and remained unchanged throughout the remaining part of the cell cycle. The content of RNA increased discontinuously during G1/early S phase and late S phase/G2, by a factor of about 3. The protein synthesis rate increased by a factor of 2.5 and the protein content doubled, both parameters discontinuously as found for the RNA content. On the other hand, the cellular volume increased continuously throughout the cell cycle and thus no close correlation between cellular volume and protein content was indicated by our results. A possible variation in the degradation rate of RNA during cell cycle is also discussed. The close relationship between RNA content and protein synthesis during cell cycle indicates regulation of protein synthesis at the transcription level.