Spinal cords of rats were exposed to either single doses of helium ions ranging from 500 to 3500 rads, or to fractionated doses of helium ions ranging from 1300 to 6530 rads using 10 equal fractions given over 22 days. Both plateau and spread-out Bragg peak regions of ionization were used. For single doses there was no difference betweenplateau or spread-out Bragg peak ionization regions for production of paralysis; thetolerance dose for production of paralysis was about 1900 rads. For the fractionation experiment, there was also no difference between plateau or spread-out Bragg peak ionization regions for production of paralysis; the tolerance dose was between 5220 and 6530rads. Comparison of dose levels producing about 10% partial paralysis indicates that about 500 rads of dose is recovered between fractions. In both single dose and fractionated experiments, latency between onset of paralysis was between 20 to 24 weeks.