Effect of dietary selenium as sodium selenite on in vivo hematological parameters of Sprague-Dawley rats was examined over a 7-month period. Dietary selenium did not alter total hemoglobin, hematocrits, erythrocyte counts, or the osmotic fragility pattern of rat blood. Selenium-excessive (1.0 ppm) rats showed slightly lower but not significantly lower methemoglobin levels than selenium-adequate (0.1 ppm) or selenium-deficient rats. Platelet counts tended to be higher in selenium-excessive rats and lower in selenium-deficient rats than in selenium-adequate rats, but the differences were not statistically significant. No clear trends were observed regarding the effect of dietary selenium on total leukocyte and differential leukocyte counts. After 7 months of dietary treatment blood glutathione peroxidase activity in selenium-deficient rats and in selenium-excessive rats was 16.8% and 142.2% of the activity in selenium-adequate rats. The results indicate that long-term selenium deficiency in rats produces no abnormal hematological parameters or any compensated hemolytic anemia in vivo.