The eryrocyte glutathione reductase (GSSG-R) activity with and without FAD supplementation (Act. A and Act B, respectively) was studied in 24 undialyzed uremic patients and 25 healty subjects. Mean erytrocyte GSSG-R activity from normal subjects was 5039+-1.43 U/g Hb (the active form). After the addition of FAD to the hemolysates, this increased to 7.79+-1.07 U/g Hb, an increase of 1.88+-0.84 U/g Hb (the inactive from) (p