The effect of magnesium citrate and magnesium oxide on urinary biochemistry and on the crystallization of calcium salts was examined in 7 normal subjects and 4 patients with recurrent calcium oxalate nephrolithiasis. When magnesium citrate or magnesium oxide was administered on an empty stomach (10 mEq. 4 times per day or 486 mg. magnesium per day for 2 weeks) urinary magnesium increased by only 77 to 79 mg. per day and urinary citrate increased by 98 to 142 mg. per day. However, urinary calcium increased by 21 to 25 mg. per day. No significant changes were noted in urinary saturation of calcium oxalate or brushite or in the limit of metastability (formation product) of these salts. However, when magnesium salts were provided with meals there were more prominent increases in urinary magnesium (by 92 to 96 mg. per day) and in citrate (by 218 to 226 mg. per day). Moreover, urinary oxalate decreased. Owing to these changes the urinary saturation of calcium oxalate decreased and the formation product increased. If magnesium citrate and magnesium oxide are to be used in the management of recurrent calcium oxalate nephrolithiasis, they should be administered with meals.