The effect of onion ingestion on red cell anti-oxidizing agents such as superoxide dismutase (SOD), catalase, glucose-6-phosphate dehydrogenase (G-6-PD), glutathione peroxidase (GPx), NADH-methemoglobin reductase and reduced glutathione (GSH) was studied in relation to Heinz body hemolytic anemia in dogs. To dogs was given the onion soup equivalent to 30g of raw onion/kg body weight once a day for three days. A few days after the initiation of feeding, there was a transient decrease of SOD, catalase and G-6-PD activities to 60%, and GSH value to 70% of their predosing values with a sudden and marked Heinz body formation. These values then began to increase when Heinz bodies were observed in almost every red cells. On day 10, when red cells carrying Heinz bodies were almost cleared from blood circulation, catalase and G-6-PD reached peak values of nearly 250% of their predosing levels. Changes of GPx and NADH-methemoglobin reductase were not correlated with Heinz body counts. Methemoglobin concentration was significantly elevated and maintained in the level of 2-3% during the presence of Heinz bodies in the red cells.