[en] A comparison was made of the biological effect on mice of irradiation at different dose rates (70, 5.5 and 1.5 cGy/min) with equally effective, with respect to lethality, doses, or with physically equal doses within the range from 1/4 of LD_50/30 to LD_99-95/30. Equally effective, with respect to lethality, doses caused similar charges in the intestinal epithelium and in the haemopoetic system. The death rate kinetics was identical with doses of LD_80-95/30 within the dose-rate range under study. The equally effective doses caused injuries, different in degrees, to critical systems, including CFUs