[en] The cell growth respone of cultured Chinese hamster ovary cells, line CHO, to 800 rads of X irradiation involves a period of division delay, followed by a period of resumed division which terminates in a plateau phase. Over 95% of the cells die eventually. No direct effects on RNA or protein metabolism are evident during the delay period. During the resumed division and beginning of plateau phases, however, a specific and relatively constant reduction in mRNA synthesis relative to messenger-like RNA and heterogeneous nuclear RNA synthesis is evidenced. The ratio of mRNA to messenger-like RNA synthesis ranges from 0.8 to 0.65 during these phases. The effect is not due to altered cell-cycle distribution, and evidence is presented to indicate that it is probably not a compensatory response to the unbalanced growth that occurs during the division delay period