Exposure of rats to a single whole-body dose of x-rays (550 or 850 rads) leads to an increased formation of liver polyribosomes at the expense of monosomes and smaller oligomers. The effect of irradiation on the polysome organization of rat liver is observed in both normal and adrenalectomized animals and is accompanied by an enhanced incorporation of <tex-math>${}^{14}{\rm C}\text{-orotic}$</tex-math> acid into the liver cytoplasmic RNA; 40 minutes after isotope injection the specific activity of the smaller native ribosomal subunit is 2.0 to 2.5 times as high as normal. It is postulated that an excess output of native subribosomal particles, carrying peptide chain initiators, is the factor responsible for the increased polysome formation in the liver of x-irradiated rats.