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In order to study the effect of calcium and magnesium on recombination in rice, the following experiments were made. F_1 plants heterozygous for Pl and lg were grown in the culture solution with various concentrations of CaO or MgO, and the recombination value was calculated from F, generation. Besides, to know the relation between the CaO and MgO concentration of solution and these contents of plant body, the contents were analyzed in a watercultured rice variety. The effect of calcium: Low concentration of CaO resulted, as shown by a chi square for heterogeniety between the low concentration and the optimal one, in the significant increases in recombination value (Tables 2 and 4). High concentration of CaO resulted, on the other hand, in a slight increase in the value, though the increment was not significant (Table 4). The effect of magnesium: Low concentration of MgO resulted in the increases in the recombination value (Tables 3 and 5). The increment was not significant in the first experiment because of an insufficient difference in the MgO concentration, however, significant in the second experiment. But the effect of magnesium on recombination was not so greater than that of calcium. High concentration of MgO brought about the same change in recombination value as the high CaO (Table 5). The CaO and MgO contents: The CaO contents of plant body were considerably altered by the MgO concentration of solution, even if the CaO concentration was identical, however, the MgO contents were hardly affected by the CaO concentration (Fig. 1). Therefore, in discussing the effect of calcium and magnesium on recombination, this fact must be considered.