[en] The effect of instantaneous dose rate on the survival time of mice was studied for groups of LAF₁ mice receiving weekly γ radiation exposures, for duration of life, ranging from 150 to 300 R at instantaneous dose rates of 0.1, 1, 10 and 100 R/min. Two additional groups were irradiated at 100 R/min with weekly exposures of 150 and 250 R in which the dose was delivered in two equal fractions two hours apart. The range of weekly doses was selected with the anticipation that mean survival times would be in the range of 200 to 250 days, since the deaths associated with survival times in this range are due to tumours, leukaemia and degenerative diseases. The cause of death was not determined in this study since the primary purpose was to obtain survival time data from which leukaemia dose-rate studies could be designed. For comparison of the relative effectiveness of the instantaneous dose rates, the log median survival times were plotted as a function of weekly dose. The weekly dose to yield a median survival time of 200 days was determined, by interpolation, to be 228, 193 and 173 R/week for instantaneous dose rates of 0.1, 10 and 100 R/min, respectively. The results from the two groups which received the weekly dose in two fractions, separated by an interval of two hours, indicated a decrease in effectiveness equivalent to about 24% of the initial fraction. Similar comparisons between the 0.1 and 10 R/min groups and the 0.1 and 100 R/min groups indicated differences in effectiveness of 35 and 55 rad/fraction, respectively. It was concluded that the recovery kinetics operating during prolonged fractionation regimes have a dependence on instantaneous dose rate quantitatively similar to that observed in short-term LDsub(50/30) studies. (author)