Hypomagnesiuria is a common biochemical finding in patients with calcium oxalate (CaOx) nephrolithiasis. Clinical trials using Mg supplements as therapy against CaOx stones have shown mixed results. We tested the effect of Mg administration in healthy subjects under conditions of controlled urine pH (UpH) on urinary Ca excretion rate (UCaV) and CaOx saturation. This is a 4-phase, double blind, placebo-controlled, metabolic crossover study performed in healthy volunteers. Mg lactate (MgLact2) was used as Mg supplement. High UpH and low UpH were achieved by administration of potassium citrate (K3Citrate) and ammonium chloride (NH4Cl), respectively, with potassium balance maintained by KCl. Eight participants completed 4 phases of study. The interventions successfully modulated 24-h UpH (7.0 ± 0.4 vs. 5.7 ± 0.6 in high vs low pH phases; P<0.001). Administration of MgLact2 increased UMgV [175.8 ± 40.2 vs 93.4 ± 39.7 mg/day (7.2 ± 1.7 vs 3.8 ± 1.6 mmol/day), high vs low Mg phase; P<0.001], and increased pH both at low (5.6 ± 0.5 to 5.8 ± 0.7; P = 0.02) and high UpH (6.9 ± 0.4 to 7.0 ± 0.3; P = 0.01). At a given urine pH, Mg supplementation marginally increased UCaV, but did not alter UOxV or CaOx saturation. Provision of an alkali load significantly lowered UCaV and saturation of CaOx at any level of UMgV. Compared to changes in UMgV, changes in UpH play a more significant role in determining urine CaOx saturation in healthy subjects. Mg supplements are likely to reduce CaOx saturation if they also raise urine pH.