Two experiments were conducted to investigate the effect of riboflavin deficiency on the glutathione peroxidase system in erythrocyte, liver and muscle of the baby pig. In the first study, 15 one-week old pigs were assigned to three dietary regimens for 6 weeks. Pigs were either fed riboflavin-free (R-) or riboflavin-supplemented (25 mg/kg diet) diets. One-half of the supplemented pigs were pair-fed to the deficient pigs. At the end of the experiment, blood was sampled, pigs were killed and organ samples taken. Erythrocyte (RBC) active glutathione reductase (GR) was reduced by 75% among R- pigs without any change in RBC reduced glutathione. Most importantly, hepatic glutathione peroxidase (GSH-Px) activity and selenium (Se) concentration were markedly reduced by riboflavin deficiency while hepatic GR activity was unaltered. Muscle GSH-Px was reduced significantly but muscle Se was not. In the second study, 16 three-week old pigs were assigned to graded dietary levels of riboflavin (0, 2, 4 and 10 mg/kg) for 2 or 4 weeks. At 2 and 4 weeks, one-half of the pigs were killed, and blood and tissues taken. Hepatic GSH-Px was again depressed when pigs were fed the 0 mg/kg riboflavin diet, but was essentially the same whether 2, 4, 10 mg riboflavin/kg diet was fed. Muscle GSH-Px activity was more variable and did not respond to riboflavin deficiency in this study. RBC percent active GR plateaued between 2 and 4 mg riboflavin per kg diet.