To evaluate the effects of long-term voluntary resistance exercise (climbing) compared with aerobic exercise (swimming) on iron status in severely (4 mg Fe/kg diet) and mildly (18-29 mg Fe/kg diet) iron-deficient rats, we trained male Wistar rats for 8 wk to climb a wire-mesh tower (phi20 cm x 200 cm, CLIMB) and to swim in a plastic pool (phi50 cm x 50 cm, SWIM). These rats were compared with sedentary (SED) rats. After the experimental period, blood hemoglobin level, plasma iron concentration, and transferrin saturation were significantly lower in the 4 mg Fe/kg diet rats than in the 18, 29, and 40 mg Fe/kg diet rats. In 4 mg Fe/kg diet rats, the hemoglobin level was significantly higher in the CLIMB group than in the SED and SWIM groups. On the other hand, neither exercise affected iron status in mildly iron-deficient rats. Bone marrow delta-aminolevulinic acid dehydratase activity was significantly higher in the CLIMB group than in the SWIM and SED groups. These results suggest that long-term resistance exercise is more effective than aerobic exercise in improving blood hemoglobin concentration in severely iron-deficient rats, probably because of an increase in heme biosynthesis. Resistance exercise may be a useful therapy for iron-deficient anemia.