Essentiality of selenium (Se) for Japanese quail, Coturnix coturnix japonica, was confirmed using a formulated semipurified low-Se diet (basal) (0.05 ppm). Selenium-deficiency symptoms appeared in quails on this diet within 15 d, which corresponded to low levels of hemolysate glutathione peroxidase (GSH-Px) activity. Selenium administration at 0.05 and 2.0 ppm levels resulted in an increase of hemolysate GSH-Px activity by 64 and 116%, respectively, in both short- and long-term experiments. Growth over a 2-mo period increased the hemolysate GSH-Px activity by 120% at each level of dietary Se. A differential response was exhibited by hepatic mitochondrial and soluble GSH-Px activity to Se supplementation, the former increasing progressively with increments of Se at 0.05, 2.0, and 4.0 ppm by 45, 70 and 150%, respectively. The soluble GSH-Px activities of tissues, such as liver, kidney, and testis, and RBC membrane-bound activity remained unchanged in long-term studies at different levels of Se. Replenishment of Se to quails maintained on low-Se diets reflected no change in RBC membrane-bound and liver-soluble GSH-Px activities, although the activity in hemolysate increased consistently with Se. The GSH-Px activity in hemolysate was restored to the levels comparable to those of long-term studies only at Se administration at the 2.0-ppm level. The differential response of mitochondrial and soluble GSH-Px activities to Se and other related observations on mitochondrial functions suggest an additional role for Se in mitochondrial membrane processes and glutathione-related metabolic regulations. © 1986 The Humana Press Inc.