The effect of Se depletion on mammary tumorigenesis following dimethylbenz[a]anthracene [DBA] administration was examined in female Sprague-Dawley rats that were fed different levels and types of fats. Four basal diets deficient in Se were used: 1% corn oil; 5% corn oil; 25% corn oil; and a diet containing 1% corn oil and 24% hydrogenated coconut oil. Comparable Se-adequate diets were obtained by adding 0.1 ppm of Se to each basal diet. In animals that received adequate Se, an increase in fat intake was accompanied by an increased tumor incidence when corn oil was used in the diets. A high saturated fat ration was much less effective in this respect. Only in those rats that were maintained on a high polyunsaturated fat diet (25% corn oil) did Se depletion result in a further increase in tumor incidence and tumor yield. Such an augmentation was not observed in animals given a 1 or a 5% corn oil ration or a diet rich in saturated fat. Regardless of Se status, almost all of the tumors formed were adenocarcinomas. An enhancement of tumorigenesis as a result of Se deficiency in rats fed the 1% corn oil regimen was detected provided a high dose of DBA was used, suggesting that alterations in DBA metabolism might be involved under this condition. The antioxidant property of Se is discussed as a possible mechanism by which Se protects against tumorigenesis, especially in animals with a high polyunsaturated fat intake.