The effects of 300 kV X-rays, 60Co gamma-rays, 43 MV X-rays and 43 MeV electrons on total cell RNA and 45 S pre-r RNA were investigated in Novikoff Hepatoma ascites cells. Six hours after local irradiation of the animals with 600 rad, the tumour cells were labelled in vivo for 30 and 60 min. The specific activities of high-molecular-weight RNA were influenced differently. This may be due to divergent effects of the applied radiation qualities at the level of transcription and transformation (processing). In addition, these radiation effects are mediated by the adrenal glands in a rather complex manner. Therefore, the results presented in this study support the suggestion that an in vivo labelling system is unsuitable for the evaluation of quantitative radiation effects on RNA.