The rat tail has been used as a model system to study the necrosis of an organized tissue following fractionated doses of collimated D-T neutrons or 290 kV X rays. RBE values for tail tolerance - 10 per cent of tails necrosing after the early skin reactions - rise from about 1-7 (single doses) to about 3-1 (16 fractions in 22 days). Neutron tolerance doses are almost independent of fractionation from 2 to 16 fractions. The tissues at risk are shown to be rather hypoxic. Early skin reaction levels can be used to predict the fraction of tails that will necrose. Early peak reactions for a given fraction of necrotic tails were slightly higher for neutrons than for X rays, and this difference was consistent for all the dose fractionation schedules employed.