ABSTRACT. The effect of selenium (Se) supplementation to the diet on the plasma progesterone concentration was investigated in non-lactating and non-pregnant cows. Italian ryegrass wafers and concentrates, with or without 0.5 ppm of Se, were fed to cows at a maintenance level. The plasma Se concentrations in the each treatment were 0.047 ppm (-Se) and 0.081 ppm (+Se), respectively. Se supplementation did not affect the length of the estrous cycle, but it did increase the concentration of plasma progesterone in the estrous cycle (P<0.001). These results suggest the possibility that Se contributes to the progesterone production of corpus luteum.