Sulfite oxidase catalyzes the oxidation of sulfite to sulfate. To investigate whether or not sulfite oxidase activity (EC 1.8.3.1) is regulated by the amount of sulfur from dietary protein or excess methionine, we fed rats diets containing 5, 10, 20 and 50% casein with or without excess methionine and measured sulfite oxidase activity in liver and intestinal mucosa. Hepatic sulfite oxidase activity was significantly lower in rats fed 5 or 10% casein diets and significantly higher in rats fed 50% casein than in rats fed the control diet containing 20% casein, but activity did not change in response to the addition of methionine at any level of protein. Sulfite oxidase activity in the intestinal mucosa was only 5% of that seen in liver and did not change in response to dietary protein or methionine. Activity did not change in rats fed low iron diets (5 mg Fe/kg diet) at any level of protein tested or in response to glycine. These results show that sulfite oxidase activity can adapt to different levels of dietary protein but is unaffected by the level of methionine, total amino nitrogen or iron in the diet.