The influence of tetrachlorodecaoxide (TCDO) on the chronic radiation damage after local irradiation of the rat colon was examined when given at different times and at different doses. A tetrachlorodecaoxide treatment before the acute period resulted in an extension of the latency period. When tetrachlorodecaoxide was given by the highest dose of 0.8 ml WF 10 at the onset of the chronic phase we observed an increase in the survival rate. Endoscopically and histologically we did find demonstrable signs of an improved healing of the mucosa.