Selenium (Se) toxicity and utilization was evaluated in hamsters fed casein- and torula yeast-based diets. 4-week-old hamsters received semipurified diets for 21 days. In experiment I diets were supplemented with either 0.25, 10, 20, 40 or 80 ppm Se as sodium selenite (SS) and in experiment II diets were supplemented with 0.1 5.0 or 10.0 ppm as SS or selenomethionine (SM). Blood and tissue Se concentrations and glutathione peroxidase (GSH-Px) activity were measured at the termination of the feeding period. In both studies growth rate was depressed and food consumption decreased in hamsters given diets supplemented with 10 ppm or greater SS. Mortality associated with Se toxicity occurred only in females fed the 80 ppm Se-supplemented diet. Whole blood and tissue Se concentrations rose with increasing dietary Se and occurred up to the 80 ppm Se level in blood. Liver, kidney and lung Se concentrations were higher in hamsters fed SM than for those fed SS. Plasma GSH-Px activity was not significantly affected by increasing dietary Se levels, and hamsters fed dietary Se levels above 10 ppm did not have increased erythrocyte GSH-Px activity associated with increased blood Se concentrations. Liver GSH-Px activity was higher in SM-fed hamsters. The results suggest that dietary Se, fed as SS, becomes toxic for Syrian hamsters at levels of 10 ppm and above.