This study examined the influence of the oxygen-derived free radical removing agents allopurinol and dimethyl sulphoxide (DM SO) on the occurrence of hepatic metastases and on the survival rate in the rat with 1,2-dimethylhy-drazine (DMH)-induced colonic tumours. At 10 weeks of age, rats were sub-cutaneously injected every week with 10 mg per kg body weight of DMH for 28 weeks. This produced colonic carcinoma in 80% of animals. The rats that were at this stage continued on their drinking water developed multiple hepatic metastases within 3 months and died at the age of 14.9 ± 0.3 months (mean ± SEM). Administration of 1,2 or 5% allopurinol or DMSO for drinking after production of the colonic tumours prevented the development of hepatic metastases 3 months later and significantly (p < 0.01) extended survival to at least 22.1 ± 0.1 months of age (mean ± SEM). The results suggest that in the rat with colonic carcinoma, removing oxyradicals impairs the development of hepatic metastases and prolongs survival.