Spinach was evaluated for its bioavailability of magnesium in the experiment with magnesium-deficient rats. The effect of oxalic acid on absorption of dietary magnesium was also examined in the same experiment. After there were significant differences in the body weight of the rats between the control group and the magnesium-deficient group, and after the number of dead rats increased, the magnesium-deficient rats were divided into six groups. They were pair-fed for 8 days on the magnesium-deficient diet, magnesium-deficient diet supplemented with raw powdered spinach (R-sp), boiled powdered spinach (B-sp), or fried powdered spinach (F-sp), control diet supplemented with oxalic acid (Ox-C), and control diet (+Mg). On the 10th day, there was no significant difference in the food intake of the rats between the control group and magnesium-deficient group. However, the body weight, and body weight gain of the rats increased more significantly in the control group than in those of the magnesium-deficient group. Also, the contents of calcium and phosphorus in the liver and kidneys, and serum calcium content increased significantly in the magnesium-deficient rats compared with those of the control rats. However, the serum magnesium content decreased significantly in the magnesium-deficient rats. An especially large amount of calcium was accumulated in the kidneys of the magnesium-deficient rats. At the end of the experimental period, there were no significant differences in the food intake, body weight and body weight gain of the rats among the control group and each of the spinach-added groups. The body weight and body weight gain of the Ox-C rats decreased significantly in comparison with those of the control group and each of the spinach-added groups. Although, there were no significant differences in the concentrations of serum minerals (Mg, Ca and P) among each of the groups, kidney magnesium, calcium and phosphorus, and liver magnesium and phosphorus were significantly higher in each of the spinach added groups than those of the control, Ox-C and +Mg groups. A large amount of calcium was accumulated in the kidneys of the rats fed on the R-sp, B-sp, F-sp and Ox-C diets. However, the kidney calcium of each of the spinach-added groups markedly decreased in comparison with kidney calcium of the magnesium-deficient rats on the 10th day, when the magnesium-deficient rats were separated. There was no significant difference in the magnesium content of the left tibiae among each of the spinach-added groups. Also, the magnesium contents of the left tibiae of each of the additional groups did not reach the level of those of the control rats. The contents of calcium and phosphorus of the left tibiae were not significantly different among any of the groups except for both the R-sp and Ox-C groups, and decreased significantly in the R-sp and Ox-C groups compared with those of the other groups. A highly positive correlation between bone calcium and bone strength was not observed in this study; the breaking force of the left femurs of the B-sp and F-sp rats increased significantly in comparison with that of the Ox-C group. The rate of magnesium absorbed by the rats receiving the control, R-sp, B-sp, F-sp, Ox-C, and +Mg diets was 88.9, 80.2, 88.4, 90.4, 88.1, and 87.7%, respectively. The rate of apparent absorption of calcium from the control, Mg-deficient, R-sp, B-sp, F-sp, Ox-C and +Mg diet was 87.0, 84.1, 57.3, 66.4, 66.2, 53.3 and 83.5%, respectively. The data indicate that oxalic acid remained in spinach after cooking of boil or frizzle was not deleterious to magnesium availability, and that spinach is one of the most promising sources of magnesium.