The median survival time of experimental animals exposed to Xor y-ray dosages ranging from 100 to more than 100,000 r has been determined by a number of investigators (1-8). In general there is excellent agreement as to the shape of the doseresponse curve and as to the fact that this curve shows several distinct segments which may be related to radiation effects on separate body regions. To our knowledge, no studies have been reported on the effects of massive doses of neutrons on survival time. In the dose range of less than 1000 rep, neutrons are, in general, more effective than Xor y-rays for the production of radiation damage in mammalian systems (9). It is not known whether this greater effectiveness holds at a higher dose range. The present study was undertaken to determine whether or not the dose-response curve for median survival time as a function of dose of thermal and fission neutrons is similar to that for Xor --ray exposures and to determine whether or not the relative effectiveness of neutrons varies as a function of total dose over a wide dose