The activities of superoxide dismutase (SOD) and several proteases were measured in kidney of mice treated with allopurinol in order to elucidate the mechanism of prophylactic action of allopurinol against chemotherapy-induced stomatitis. The following results were obtained. Following 3 day administration of allopurinol 20 mg/day per os (Group C), the concentrations of allopurinol and oxipurinol in the renal tissue were 203.9 +/- 52.1 and 1141.7 +/- 194.8 micrograms/g, respectively. The SOD activity was significantly lower in Group C than in the untreated control group (p < 0.01). The enzyme activities of papain and trypsin were suppressed in Group C. However, the other proteases tested were not affected by the administration of allopurinol, indicating only weak anti-protease action of allopurinol. These results suggest that allopurinol may be effective to prevent chemotherapy-associated stomatitis via both direct and indirect actions to oral mucosa, that include inhibitory actions on xanthine oxidase as well as protease.