The effect of selenium intake on the development of pancreatic cancer was investigated in female Syrian golden hamsters. Four-week-old hamsters were divided into 2 groups according to the selenium level in their drinking water and were fed a purified diet containing less than 0.05 ppm selenium. Starting 4 weeks later, groups received 10 s.c. injections at weekly intervals of N'-nitrosobis(2-oxopropyl)amine (BOP) dissolved in saline, while controls received saline alone. When the animals were killed 18 weeks after the last injection, palpable tumors were less frequent in the high-selenium group than in animals receiving low-selenium supplement, the numbers of histologically diagnosed cancerous lesions also being significantly reduced by high selenium intake. The selenium level and glutathione peroxidase activity in serum and pancreas were significantly greater in the high-selenium group. Moreover, selenium levels and glutathione peroxidase activity were both significantly higher in tumor-bearing tissue. The results suggest that glutathione peroxidase is involved as an intermediate factor in prevention of carcinogenesis by selenium.