Twelve hundred and fifty-two pairs of male NEDH rats were parabiosed; 1 partner received a single dose of 1000 R X-radiation and the other rat was shielded with Pb. Eleven hundred and twenty pairs survived over 200 days and constituted the basis of this report. Twenty-five irradiated partners developed malignant tumors of the prostate, 19 of which were adenocarcinomas. One adenocarcinoma appeared in the shielded partners and 1 in a control group of 586 parabiosed and single rats. The long-term effect of a single dose of 1000 R whole-body X-radiation is weakly carcinogenic for the rat prostate.