PURPOSE:This study was designed to determine the cancer prevention and therapeutic effects of selenium on rats treated with 1,2‐dimethylhydrazine (DMH).METHODS:One hundred sixty Spraque‐Dawley male rats were divided into seven groups and received 20 mg/kg/week DMH, subcutaneously for 20 weeks. Two different dosages of selenium (8 and 4 ppm) were administered to the rats through drinking water during DMH treatment (B and C groups) or one month before and during DMH treatment (D and E groups). The rats of Groups A (control group), B, C, D, and E were killed immediately after the last DMH injection. The incidence of intestinal cancer in each group was compared. Eight ppm selenium was also administered to rats after DMH treatment (Group F), and survival times were observed and compared with Group G (treated with DMH only).RESULTS:Rats of Groups B and D received 8 ppm selenium and had a significantly decreased incidence of intestinal cancer (from 65.8 percent (Group A) to 33.3 percent (Group B) and 27.8 percent (Group D);P=0.0225 and 0.0038). Rats receiving 4 ppm selenium had a relatively decreased incidence of intestinal cancer (from 65.8 percent (Group A) to 44.4 percent (Group C) and 47.1 percent (Group E) butP>0.05). Survival time of Groups F and G showed no difference.CONCLUSIONS:Eight ppm selenium providedviadrinking water has a significant intestinal cancer prevention effect in the presence of a high dose of DMH (20 mg/kg×20 weeks), and the cancer therapeutic effect of selenium is doubtful in this animal model.