The effects of body iron stores on diquat (DQ)-induced toxicity were examined in male Fischer-344 rats, which are sensitive to this herbicide. The rats (5 weeks old) were fed diets containing 40 (lower iron storage [LIS] group) or 320 ppm iron (higher iron storage [HIS] group) for 5 weeks. The concentrations of nonheme iron and ferritin in the liver and kidney were significantly higher in the HIS group than in the LIS group (P<0.0001), although there was no significant differences between the HIS and LIS groups in hematological parameters, including red blood cell count, hemoglobin concentration, and mean corpuscular volume. Three hours after administration of 0.1 mmol DQ/kg, serum alanine aminotransferase and urea nitrogen were significantly higher than in controls (saline injection) for both the LIS and HIS groups (P<0.01), and, after DQ injection, these parameters were significantly higher in the HIS group than in the LIS group (P<0.01). When the rats were injected with 0.075 or 0.1 mmol DQ/kg, the survival time was significantly shorter in the HIS group than in the LIS group (P<0.05). These findings suggest that higher body iron stores result in more severe DQ toxicity in Fischer-344 rats.